I created custom entities, put them in custom structures and attached the structures to feature and feature rule jsons. When adding the behavior pack to a world, the structures load fine, but none of the entities. The doors and trapdoors are also sometimes flipped a certain way, so I would assume this would be true with most rotation blocks as well.

I can confirm that I included entities when exporting the structures. There are no issues with the entities. I also spawned the structures with a structure block and the entities loaded perfectly, so I can also confirm that this is primarily an issue with the features themselves.

This was all tested on version 1.16.210