maybe this is caused because the changes (new/removed tags) are not really applied to the file until you click to another file. This reminds me to my issue (already noted) where new Tags are removed (or better: keep blank because the test starts with a group without tags) when dragging files from the Finder to that new group while modifying the tags of that group.
Just sharing my experience - not sure whether it’s intended or glitches.
The inspector bar may show that new tag/s are added, but I need to hit “enter” in the tags section of the inspector bar to confirm saving all the newly added tags before switching to any other tasks (including changing the name in the same inspector bar), else the tags won’t be retained.