Well as I said, support will say anything you like to hear. Nothing they announce to fix will get fixed more likely than them saying nothing about it....
Seeing the global x90 pro getting monthly security updates means also the promise for 3 android upgrades clearly doesn't apply to the x90pp.
Just received 20.1 trial as well and all ok so far. Will post some more photos in the other thread soon.
It's fixed a big undocumented bug/problem from the last build where I wasn't able to sideload apk files and I read other users having the same issue previously.
Can you check if by chance the update restored vivo iManager and/or Vivo appstore. That would be the most likely explanation - because if they are removed you cannot sideload apk files - restoring them fixes it. Now any update by Vivo could reactivate some of their spyware/bloatware apps - and likely even in that case make them impossible to remove anymore.
Happened with phone and messages on some update - they used to be removable with adb but now this is not possible anymore. At least they weren't restored but that is a thing vivo could easily do too with a system update.