All major UE target platforms are little-endian. Saving packages with byte-swapping seems to have been broken for more than six years, when CL# 1495479 changed `FArchive::ByteOrderSerialize()` so that we call Serialize() before byte-swapping.
There can be a lot of Serialize() implementations that depend on the current behavior and expect that the data they're about to be saved is not byte-swapped. It might just be simpler to say that UE does not support byte-swapping during save, only load, and add a check() with a good error message for this case.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80853 in the post.