This bug was discovered when I had an auto-tag specification on a short string, e.g. “art”. The result auto-tagged topics with “artificial”, etc.
FWIW: the “test” feature on the auto-tag spec page works fine (for auto-tag on “art”, “artificial” does not generate a tag in the test).
The bug has probably not been noticed because it is perhaps uncommon to auto-tag on a short watched word.