I just played a session and noticed that saving didn't work properly anymore - I would enter a save name and hit save, but afterwards, the save would not appear in the list. When I tried to reload the last appearing save in the list, the game told me it detected save corruption and could not load it.
After the initial wailing and gnashing of teeth, I noticed that the last save in the list, the corrupted one was right after some milestone. I reloaded the last working save, played up to the part where that corrupted save would have been and noticed that it was right after some cutscene where the game would have been autosaving WHILE I initiated the quicksave. It seems that you can royally screw yourself if you cause overlapping save procedures, so for the foreseeable future, I will make sure that the only one initiating saves is me, no more autosaves here.
tl;dr: If you save while an autosave is being made, your save will risk being corrupted. Generally don't overlap saves in progress.
How did the 2 saves at the same time even happen? Normally, BG3 blocks all further saving while a save is already in progress. Managed to skip an autosave trigger like that before.
The save process is usually initiate save (f5 or via menu), then the noticeable part where you can't move or do anything, then you can move your characters again but if you hit esc, the buttons "save" and "load" are still greyed out. I assume that this is the interval where shenanigans can occur. A few seconds after that, the buttons become usable again and the "saved successfully" message appears on-sceen.
Instead of disabling autosave you could also give the advice to only save the game if your characters are standing still or if you are currently in a dialogue at a point where you can select an option
That should make it impossible to save the game while autosave is running
I noticed that after manually instigating a save, there's a phase where the screen is darker and you can not move anything, after that, the screen goes back to normal and you can move again for a bit but only a little time later, the game will say "save complete" or something to that effect. And in that phase where you can move (but can also stand still), if you press escape, the save and load buttons are still greyed out. I assume that that was the critical phase where starting another save would wreck something.
I can not confirm that though, but, for what it's worth, I can say that I have had no corrupted saves since I disabled saves and pay attention to not start quicksaves before the game says save successful.
I also noticed that one patch since said that creating corrupted saves had been addressed, so maybe the whole thing would be moot now anyway.