Started having this issue last week.
When I add multiple tags on the same file one after the other, the new one overwrites the previous tag.
More details I found while replicating:
-When I switch to a different file and then back, I can add a new tag with no issue. But again, the following one overwrites it
-Been able to replicate with any location and tag
-If I add multiple tags all at once (using ⌘+T) i can add as many as i want, but if I add a new one manually afterward it will erase the whole bunch
Does the following one also overwrites the previous if you switch files again before adding the next tag?