I had that problem in early access. It looks like they never fixed it. For me the stuff returned some hours later, though.
What I consider mistakes by now:
-- putting a chest within a chest within a chest
Chance is, the content of the innermost chests will go *poof* as soon as you travel to, say, the underdark. Any change of camping spot will do.
-- placing new chests anywhere that isn't the ground
Mine might have fallen through the stones. Eventually came back.
-- adding anything that isn't a chest to your camp
That stuff did not come back.
-- not regularly checking the bags/chest of the mundane you keep all your important stuff in.
And
-- keeping said bag/chest anywhere that isn't your person.
Unfortunately, going into act 3 will break it anyway if you do.
If you're lucky, your stuff may be stranded in a camping spot you've lost access to. That means it exists and might come back some hours later. I believe Bg3 works by sending copies of items into your chests, not necessarily keeping track of where the o.g. items went. That can go oopsie when it forgets your chest is supposed to have some in it. Or multiple chests at the same time.
