Same problem here. I will go in and make changes to a post in Quick Edit, and the post will be saved with my name.
This happens both with bulk Quick Edit (say, adding categories to a bunch of posts) and individual Quick Edits of posts (changing a posting date).
I’ll try more of it on a dev site, to see if I can find commonalities.
I’ll also be happy to set you up with an admin login, if you want to take a look for yourself.