Blueprint improperly marked dirty when it contains a latent node with a byte enum

In UE 5.4, when deleting actors in our levels, some blueprints will be mark dirty despite not having changed. We traced that down to FAssetRenameManager::CheckPackageForSoftObjectReferences.

The affected blueprints have the following properties

• They have soft references to actors in the level
• They have a latent node with an enum parameter in the delegate type

The following happens when an actor is deleted in the level,

• FAssetRenameManager checks if the blueprint has any soft reference to the deleted actor.
• In order to do so, it serializes the Blueprint with a custom serializer. The serialization observes a mismatch of the pin's default value with the following callstack

UK2Node_EditablePinBase::UpdateUserDefinedPinDefaultValues() Line 446
UK2Node_FunctionEntry::FixupPinStringDataReferences(FArchive * SavingArchive) Line 1078
UK2Node::Serialize(FArchive & Ar) Line 133
UK2Node_EditablePinBase::Serialize(FArchive & Ar) Line 256
UK2Node_FunctionTerminator::Serialize(FArchive &) Line 39
UK2Node_FunctionEntry::Serialize(FArchive & Ar) Line 273
FAssetRenameManager::CheckPackageForSoftObjectReferences(...) Line 1568
FAssetRenameManager::GatherReferencingObjects(...) Line 1046
FAssetRenameManager::FindSoftReferencesToObjects(...) Line 320
UUnrealEdEngine::DeleteActors(...) Line 940
UActorElementEditorWorldInterface::DeleteElements(...) Line 77
FTypedElementCommonActionsCustomization::DeleteElements(...) Line 30
FActorElementLevelEditorCommonActionsCustomization::DeleteElements(...) Line 24
UTypedElementCommonActions::DeleteNormalizedElements(...) Line 83
UTypedElementCommonActions::DeleteSelectedElements(...) Line 62
UUnrealEdEngine::Exec_Actor(...) Line 2610
UUnrealEdEngine::Exec(...) Line 725

The mismatch is caused by the fact that the default value of enum pins is handled differently between the UserDefinedPins and the Pins arrays.

• "Pin->DefaultValue" will contain the string value of the enum
• "PinInfo->PinDefaultValue" will be an empty string

The code in UpdateUserDefinedPinDefaultValues will copy the string value of the enum in "PinInfo->PinDefaultValue" and mark the blueprint dirty. Since the modified node is transient (K2Node_FunctionEntry for the output pin of the latent node) this new value doesn't get serialized so the "fixup" doesn't hold up.

• Create a UAbilityTask class with an enum parameter in the delegate type
• Create any class of blueprint which uses the new UAbilityTask node, e.g. a Gameplay Ability
• Delete any actor from a level
• Note that the blueprint has been marked dirty despite no reasonable changes to it