This is my first post here.  I found this thread when I called QB support about this issue last week.  They blindly sent me the link for this thread, assuming something here would resolve my issue!  I skimmed through and quickly found there is no resolution as yet.  The support person made sure I have the latest releases and patches, etc, but nothing she did made a difference.

We did not have this issue of the Inventory Items List constantly resizing the columns to some kind of default size before upgrading to QB 2021 (Pro Plus).  This never happened with the 2020 version, so in our experience it seems to be a "bug" that was introduced in the 2021 update.