@Zachary Whyte may I ask if this issue has been addressed with the full release?
I haven't had the chance to test yet on the full release, but I do have a Beta save (just uploaded as "Portsmouth.fm", will use better naming next time) where I used external tools to check hidden attributes (NO editing has been done with these tools whatsoever) and new signings got their hidden attributes and determination completely copied over from the player that had welcomed them to the club (3-4 in-game months after signing). Most signings have been welcomed by Brahim Diaz and Majecki and share their exact attributes. Pape Matar Sarr for example has been the only one welcomed by Godfrey and they share all their hidden attributes.
Unfortunately I do not have a save before the attribute changes, but I can keep an eye on it and share additional files later on if I find the bug still occurs.