@Martin Didn't even come close to reading the whole context there, but it may be that the tag synonym exists (which is why it's getting remapped automatically), yet the synonym hasn't been merged into the "master" tag. You can do that easily from the tag dashboard. Should address a lot of the confusion

...oh wait. You're not a moderator. Um, never mind then. You can't actually do this, then.

Well, anyway, discussed here on Meta SO.

@CodyGray Yes, I tried to explain difference between merging and synonyms to that user who asked about that.

In this case, the synonym was created by users through voting on a synonym (not by a mod), which is a synonym without merging by default.

In this specific case, on a per-site meta somebody suggested cancelling the synonym - which cannot be done after merging. So I assume that while possibility of removing the tag synonym is discussed, it is better not to merge.

Of course, somebody might observe that "this tag disappears when I enter it?" for any tag which is a synonym - regardless of whether it was merged into the master tag or not.

@Martin Correct on all counts.

Sounds like you don't need my input. :-)

Here are some links which also explain some stuff about merging which I have mentioned to the user who asked that: What are tag synonyms and merged tags? How do they work? and What happens with the tagged questions when a tag-synonym is cancelled?

@CodyGray Well, I would hope that I know thing or two about merging, since I have spent some time with discussions about tags and tag synonyms on Mathematics Meta. Still, as you can see above, I incorrectly interpreted the column "created" in the list of the tag synonyms.

I am looking at the link you provided: Why I still see the score of merged tag in my profile?

@Feeds answered that one