Ultimately it was an issue with the source build of 5.5, or one of my specialized use cases. As the issue has persisted even when setting the branch explicitly for targeting 5.5, the problem continues to persist.
If it weren’t so expensive to do so, I’d report this straight to Epic as this is very clearly a breaking bug that’s affecting a primary module. I’m otherwise out of ideas, so I’m going to mark this matter as closed.