There's been a new update that has been released by Microsoft (A system update not a BG3 update). Though it is being met with some unclear results. People are still reporting saves being lost unfortunately but I've seen one person get their save back on a Reddit post. It's hard to tell where the issue stands right now from the perspective of Xbox support but it seems as though players are still having issues.
I've personally put my playthrough on hold for now until there's confirmation about a fix.
In addition to what Salo said, and for the sake of details, while I didn't experience saves being lost I did not use Quick Resume. Some people are mentioning that keeping saves to a minimum has been helping but I've also seen mixed results with that. (IE: Two Auto Saves, Two Quick Saves, and 2-3 Manual Saves for example. And if making a new save, then deleting one of the slots and making a new one instead of overwriting.)
Fingers crossed more news comes about the issue. I've played 10-12 hours and loved my time with it so far; I'm a bit worried and scared to continue now though.
There's lots of posts on the Baldur's Gate 3 Reddit but I've found this post to be pretty consolidating with OP providing some updates and links together:
https://old.reddit.com/r/BaldursGate3/comments/18e4mq4/xbox_saves_for_baldurs_gate_3_are_being_lost/
