Possibly started in 4.21

Playing in a map results in the map's blueprint generated class being added to the JavascriptIsolate's ClassToFunctionTemplateMap. Since this is never cleaned up, changing the map results in an assert / crash that the previously loaded map is not garbage collected.

Crash log:

LogReferenceChain: (root) JavascriptIsolate /Engine/Transient.JavascriptIsolate_1::AddReferencedObjects()
LogReferenceChain:  BlueprintGeneratedClass /Game/Maps/Blank.Blank_C::AddReferencedObjects()
LogReferenceChain:   LevelScriptBlueprint /Game/Maps/Blank.Blank:PersistentLevel.Blank->Outer
LogReferenceChain:    Level /Game/Maps/Blank.Blank:PersistentLevel->Outer
LogReferenceChain:     World /Game/Maps/Blank.Blank

Fatal error: [File: ... EditorServer.cpp] [Line: 1969] 
/Game/Maps/Blank.Blank still around while trying to load new map
(Object is not currently rooted)

Local fix that seems to work for me so far, in UJavascriptComponent::BeginDestroy(),

+       else if (JavascriptIsolate)
+       {
+               JavascriptIsolate->RemoveFromRoot();
+               JavascriptContext = nullptr;
+               JavascriptIsolate = nullptr;
+       }