diff --git a/docs/changelog.md b/docs/changelog.md
index 183a98f925..7a5c63810a 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -46,6 +46,8 @@ What's changed since pre-release v1.48.0-B0228:
     [#3907](https://github.com/Azure/PSRule.Rules.Azure/issues/3907)
   - Fixed `tryGet` throwing instead of returning `null` for a property lookup against an array by @oWretch.
     [#3907](https://github.com/Azure/PSRule.Rules.Azure/issues/3907)
+  - Fixed cross-scope `existing` resource reference via symbolic name resolving to a malformed mock, breaking `concat()`/`map()` during pre-flight expansion.
+    [#3920](https://github.com/Azure/PSRule.Rules.Azure/issues/3920)
 - Engineering:
   - Bump YamlDotNet to 11.2.5.
 
diff --git a/src/PSRule.Rules.Azure/Arm/Expressions/Functions.cs b/src/PSRule.Rules.Azure/Arm/Expressions/Functions.cs
index feba3dcda8..468c93f910 100644
--- a/src/PSRule.Rules.Azure/Arm/Expressions/Functions.cs
+++ b/src/PSRule.Rules.Azure/Arm/Expressions/Functions.cs
@@ -289,21 +289,19 @@ internal static object Concat(ITemplateContext context, object[] args)
             return result.ToString();
         }
         // Array
-        else if (args[0] is Array || args[0] is JArray)
+        else if (ExpressionHelpers.TryArray(args[0], out var firstArray))
         {
             var result = new List<object>();
-            for (var i = 0; i < args.Length; i++)
+            for (var j = 0; j < firstArray.Length; j++)
+                result.Add(firstArray.GetValue(j));
+
+            for (var i = 1; i < args.Length; i++)
             {
-                if (args[i] is Array array)
+                if (ExpressionHelpers.TryArray(args[i], out var array))
                 {
                     for (var j = 0; j < array.Length; j++)
                         result.Add(array.GetValue(j));
                 }
-                else if (args[i] is JArray jArray)
-                {
-                    for (var j = 0; j < jArray.Count; j++)
-                        result.Add(jArray[j]);
-                }
             }
             return result.ToArray();
         }
diff --git a/tests/PSRule.Rules.Azure.Tests/PSRule.Rules.Azure.Tests.csproj b/tests/PSRule.Rules.Azure.Tests/PSRule.Rules.Azure.Tests.csproj
index 2c2e399493..66a4057764 100644
--- a/tests/PSRule.Rules.Azure.Tests/PSRule.Rules.Azure.Tests.csproj
+++ b/tests/PSRule.Rules.Azure.Tests/PSRule.Rules.Azure.Tests.csproj
@@ -320,6 +320,12 @@
     <None Update="Tests.Bicep.43.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="Tests.Bicep.44.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+    <None Update="Tests.Bicep.45.json">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
     <None Update="Bicep\SymbolicNameTestCases\Tests.Bicep.1.json">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
diff --git a/tests/PSRule.Rules.Azure.Tests/TemplateVisitorTests.cs b/tests/PSRule.Rules.Azure.Tests/TemplateVisitorTests.cs
index 04678391fc..26d2b940cd 100644
--- a/tests/PSRule.Rules.Azure.Tests/TemplateVisitorTests.cs
+++ b/tests/PSRule.Rules.Azure.Tests/TemplateVisitorTests.cs
@@ -1417,4 +1417,31 @@ public void ProcessTemplate_WhenNullableIndexFromEnd_ShouldReturnExpectedValue()
         var secondLastPart = secondLastPartOutput["value"].Value<string>();
         Assert.Equal("eastus", secondLastPart);
     }
+
+    /// <summary>
+    /// Test case for https://github.com/Azure/PSRule.Rules.Azure/issues/3920
+    /// </summary>
+    [Fact]
+    public void ProcessTemplate_WhenExistingResourceReferencedBySymbolicName_ShouldReturnMock()
+    {
+        _ = ProcessTemplate(GetSourcePath("Tests.Bicep.44.json"), null, out var templateContext);
+
+        Assert.True(templateContext.RootDeployment.TryOutput("addressPrefixes", out JObject addressPrefixesOutput));
+        Assert.Equal(JTokenType.Array, addressPrefixesOutput["value"].Type);
+
+        Assert.True(templateContext.RootDeployment.TryOutput("combinedAddressPrefixes", out JObject combinedAddressPrefixesOutput));
+        Assert.Equal(JTokenType.Array, combinedAddressPrefixesOutput["value"].Type);
+
+        Assert.True(templateContext.RootDeployment.TryOutput("mappedAddressPrefixes", out JObject mappedAddressPrefixesOutput));
+        Assert.Equal(JTokenType.Array, mappedAddressPrefixesOutput["value"].Type);
+    }
+
+    [Fact]
+    public void ProcessTemplate_WhenExistingResourceReferencedByResourceId_ShouldReturnMock()
+    {
+        _ = ProcessTemplate(GetSourcePath("Tests.Bicep.45.json"), null, out var templateContext);
+
+        Assert.True(templateContext.RootDeployment.TryOutput("combinedAddressPrefixes", out JObject combinedAddressPrefixesOutput));
+        Assert.Equal(JTokenType.Array, combinedAddressPrefixesOutput["value"].Type);
+    }
 }
diff --git a/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.44.json b/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.44.json
new file mode 100644
index 0000000000..636be037fe
--- /dev/null
+++ b/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.44.json
@@ -0,0 +1,29 @@
+{
+  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
+  "languageVersion": "2.0",
+  "contentVersion": "1.0.0.0",
+  "resources": {
+    "vnet": {
+      "existing": true,
+      "type": "Microsoft.Network/virtualNetworks",
+      "apiVersion": "2023-11-01",
+      "subscriptionId": "[subscription().subscriptionId]",
+      "resourceGroup": "other-rg",
+      "name": "vnet1"
+    }
+  },
+  "outputs": {
+    "addressPrefixes": {
+      "type": "array",
+      "value": "[reference('vnet').addressSpace.addressPrefixes]"
+    },
+    "combinedAddressPrefixes": {
+      "type": "array",
+      "value": "[concat(reference('vnet').addressSpace.addressPrefixes, createArray())]"
+    },
+    "mappedAddressPrefixes": {
+      "type": "array",
+      "value": "[map(reference('vnet').addressSpace.addressPrefixes, lambda('prefix', lambdaVariables('prefix')))]"
+    }
+  }
+}
diff --git a/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.45.json b/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.45.json
new file mode 100644
index 0000000000..9154ef5f62
--- /dev/null
+++ b/tests/PSRule.Rules.Azure.Tests/Tests.Bicep.45.json
@@ -0,0 +1,11 @@
+{
+  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
+  "contentVersion": "1.0.0.0",
+  "resources": [],
+  "outputs": {
+    "combinedAddressPrefixes": {
+      "type": "array",
+      "value": "[concat(reference(extensionResourceId(format('/subscriptions/{0}/resourceGroups/{1}', subscription().subscriptionId, 'other-rg'), 'Microsoft.Network/virtualNetworks', 'vnet1'), '2023-11-01').addressSpace.addressPrefixes, createArray())]"
+    }
+  }
+}