If you create a BP validator it will not function until you reboot the editor.
I believe this is because BP validators are only loaded in UEditorValidatorSubsystem::RegisterBlueprintValidators() which is called only via UEditorValidatorSubsystem::Initialize(FSubsystemCollectionBase& Collection).
Would be great if BP validators were added to the validation system on creation to save content creators from thinking their validator isn’t working to only have it ‘just work’ after a reboot.