I have seen this too, closing and restarting Agenda fixed it for me. I may have been on an older version though, I got a message about being out of step with my ioS version. I am now upgraded to version 20.0 and not seen it for a while
I seemed to trigger this today simply by searching for a tag and then clicking “Filter Notes” in the search dialogue. Although then restarting and doing the same thing again didn’t trigger it…