The problem is xbox firmware and unfortunately they (xbox) haven't updated past firmware 2921 yet.
The only info we have is they are looking at it. The link for the news page re this is
https://news.xbox.com/en-us/recent-news/?_categories=insiderand the last post that acknowledges the issue is from 14th Dec. "Xbox Insider Release Notes – Alpha (2402.231212-2200)"
So it may be unlikely we'll see a fix until the New Year
(Link to Alpha Notes:
https://news.xbox.com/en-us/2023/12/14/xbox-insider-release-notes-alpha-2402-231212-2200/)
Specific Quote about getting it fixed from the Article:
"Fixes to address an issue that could cause unexpected loss of saved progress in
various titles."
So it's out of Larian's hands now. We *must* wait for Xbox to return from holidays.
Personally, I have lost 8x 6-10hr files that cannot progress past Act 1 (bc I need to sleep lol :p)
I am losing enthusiasm for going thru the prologue, they don't let you skip it either.
The best thing to do is, after the game saves are lost, to use the Xbox function of "Report Error"
Perhaps if they are flooded with reports of this issue they'll focus on it?
Not many people do this anyway but it could help.