in the selector @a, tag=!<something> has failed at least 2 times that I know of.

1.  my admin could not pass a stairway to get to his room (2021-06-21)

2. I got kicked out of my command block room (2021-06-23)

In both cases am using it inside of a Command Block to protect an area from non-admin.  These are 2 different command blocks and 2 different areas but within same ticking area

The tags are consistent, assigned when realm was started and there is nothing that can remove it except for my manual command.