Unreal engine 5.3 - Same thing happening. I have been searching for a fix or solution today for a few hours. This post was Aug. 7, it its now September. I saw another post a day ago that had an Epic Staff post this for verification.
Nitro 5 - 32gb ram - 3060 rtx, 1tb sdd, ryzen 5800 H
GFX Driver version Nvidia - 537.42
Loading up a projects - Crashes on SAVE ALL - or save.
content / Assets do not always “save” potential for corruption.
I will try the bottom right - “unsaved button” you were describing.