Have you tried the opposite? I mean, if a UI reset didn’t work and the stock UI won’t function properly (?) then what happens when you do install an addon to manage your bags? Try that and see what happens.
I mean, it’s better than nothing, right? If an addon resolves things then maybe you can play until you get an answer to your ticket (absolutely not likely to include instructions on how you can personally fix this other than standard stuff like a UI reset, Scan and Repair or even a re-install.)
Try Adibags, it still has some minor issues but after poring over their discussions on Github for the past week or two, I’ve not seen one person complaining about bags missing or disappearing.
https://www.curseforge.com/wow/addons/adibags
Not suggesting you permanently use the Addon, and not offering an opinion on whether this is better or worse than a stock UI. That’s moot here.
Just trying to get you to be able to play and put more information in your hands when it comes to discussion of the bug. More info is never detrimental to a troubleshoot. If it works with addon XYZ but not the stock UI, that’s going to be a boon to those trying to figure out this issue.
Good luck!