I also have this problem in 5.4.1. I made a copy of my 5.2 project (which packages fine), tried to package, and I get about ~200 of the “Content is missing from cook” errors that look similar to this:
UATHelper: Packaging (Windows): LogCook: Error: Content is missing from cook. Source package referenced an object in target package but the object was stripped out of the target package when saved.
UATHelper: Packaging (Windows): Source package: /Game/Blueprints/Story/StoryInteractable_Core_MistVault
UATHelper: Packaging (Windows): Target package: /Game/Blueprints/Story/StoryInteractable_Core
UATHelper: Packaging (Windows): Referenced object: /Game/Blueprints/Story/StoryInteractable_Core.StoryInteractable_Core_C:BlockerCapsule_GEN_VARIABLE