Plugins that provide blueprint-accessible libraries generally need to be set the PreDefault LoadingPhase so they can be properly loaded before game-specific data that loads early in the Default loading phase. If a blueprint that is loaded early like GameInsatnce tries to use these functions, they could fail to load with errors like
"VerifyImport: Failed to find script package for import object 'Package /Script/ModularGameplay'"
The main new plugin that appears to be broken is ModularGameplay, but older plugins like Synthesis or other new plugins may also need to be fixed. The general way to fix this is to open the uplugin file and change the LoadingPhase for all modules with BP functions to PreDefault
There is no simple repro with internal projects. The general repro steps are to make native class in a game module reference a blueprint that calls functions on the component extension manager subsystem
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-158125 in the post.