I can filter references by clicking on a tag to show only entries containing this tag. If I now do a new search, such that the results do not contain the selected tag, I cannot deselect the (now disabled) tag. I can clear the tag selection using the context menu for the tag search field - I can also clear my search, deselect the tag by clicking on it again (to toggle selection) and re-run the search.
But it is not possible to interact with the selected-but-inactive tag. This is unexpected behaviour. Although not being able to select a tag that is not present in the search results is a sensible limitation, I feel that the ability to _deselect_ a tag should be an option whether the tag is in the search results or not.