There is no bug, just operator error. When you export from AE you must export with fields, upper. Doesn’t matter if the original material is progressive, it still needs to be in an interlaced quicktime if it is going to be put into an interlaced sequence in FCP which is obviously what is happening.AE can do this. I rant and rave at graphics people constantly until they get that setting right. When they do it works perfectly. I am nearly always working in 1920 x 1080 50i
The reason 720 seems OK is that it is a progressive sequence in FCP. If you want to throw away resolution and work in 720 instead of getting the AE export right, then that is one option.