I tried to remove the "constructive algorithms" tag from 1704F - Игра-раскраска because the way I see it, there is nothing "constructive" about that problem. Within a minute, it reappears.

The first impression one might have is that I got into an "edit war" with someone who adds it back. However, I tried it 3 different times during the day (each time multiple times) and it has reappeared quickly every time. I don't believe that out of the few hundred people who have tag edit access on that problem, out of whom only a fraction care about the tags, out of whom only a fraction think the problem is "constructive", someone would instantly notice that the tag was gone and readded it every time.

There is the possibility that it is done by a bot, but it seems unlikely that someone would write a bot like that seeing as it serves no purpose and is not that trivial to do.

The most likely interpretation still seems to be that removing the tag is actually broken. My theory is that the server caches that the tag is supposed to be removed but never actually removes it from the database, so when the cache is invalidated or reloaded, the tag reappears.

Some more experiments:

• I can add and remove new tags.
• I asked someone else to add a bogus tag to a problem. I was able to remove that.
• I added a bogus tag to F and let it sit there for an hour or so. I was able to remove that.

In my opinion, no problem in that contest is "constructive". D might be under a very generous interpretation.