We are using about 20 or so different tags on our site (which all show up and work on /tags), but for some reason only some tags autocomplete when typing into tagging box for topics.
Ok, so we thought maybe the autocomplete on the tag search isn’t working, so we manually type the tag (which should work even if the tag doesn’t exist) and hit save, but the tag just disappears right away after saving, and checking the revisions shows:
tags changed:  →
All the tags do seem to be POSTing correctly, and I even tried directly with the API, but only the existing tags that autocomplete and new tags will stick, while the existing tags that don’t autocomplete just disappear without any errors.
The weird thing is that hash-tags within posts still works perfectly (autocomplete and everything) for all our tags, so it’s just something wrong on the topic tags.
Any help would be appreciated.
Edit: Just noticed that these “missing tags” are also absent from that tag filter dropdown.
Also noticed that all of these tags have been used over 900 times.