notmyfault, I think that this is not a regression. On Jenkins 2.332..2 I see the same behavior:
Narrow the Jenkins 2.332.2 search by entering 'mercurial' in the filter field
Clear the Jenkins 2.332.2 search by clicking the "x" in the filter field
After clearing the search, the list is still only showing "Mercurial", even though the list is no longer filtered.
I agree that it is a bug and it is surprising, but I don't think that it is a regression, at least not a recent regression.
I confirmed that the same behavior is seen in the 2.344 pre-release build that I created from the current master branch.
For reference, the issue does only apply to the "Installed" tab. The correct behavior can be reproduced by applying the steps to the "Available" tab.