Issue Summary

If I filter issues per one assignee (john) on the board and reassign all issues from this assignee (john) to another user (mary), the option to filter through this assignee (john) disappears on the quick filters, however, the board is still filtering by them (john).
Screen+Recording+2021-06-28+at+03.07.25+PM.mp4

Steps to Reproduce

1. Have issues assigned to one user
2. Quick filter on the board to only see issues assigned to this one user
3. While the filter is active, change the assignee on the issues to another user
4. The issues will vanish from the board (as there are no issues assigned to this one user)
5. The quick filter option to filter through this one user also vanishes
6. The board is still filtering by this one user, though they are no longer a filter option

Expected Results

Although there are no issues to be seen on the board, as the board is still filtering by that one user, it should show as a filter. 

Actual Results

The board shows as empty and having no filters applied to it (but there is).

Workaround

Manually click Clear filters to remove all filters from the board or refresh the page