I assume that most likely this was just an oversight - but since it was a result of a moderator's action, it is probably better to ask than just remove the tag singlehandedly.1
Question: At the moment there are two separate tags open-problem and open-problems. (The tag open-problem is on a single question.) Is it just a mistake, or does MO have two tags with a very similar names on purpose? If this is intentional, what is the distinction between the two tags?
As far as I can tell, the tag open-problem was recreated when a moderator recently undeleted this question: Alternating colors on a line: infinitely often or converge? This tag is still listed among the new tags at the moment.
The history of this tag is a bit complicated. According to Wayback Machine, it existed back in 2017. To the best of my recollection,2 at some point in 2017 a moderator merged the tag open-problem-list into this tag and renamed the tag to open-problems. (However, the tag open-problem remained on the deleted questions - merging only influences questions which are not deleted.)
1After all, if this question is not needed, it can be simply deleted. But I did not want simply revert an action taken by a moderator. (One additional advantage of having this question around is that it documents that a tag can be (re)created by undeleting a question which used to have that tag before it was revmoved/renamed.)
2I am unaware of some place where merging of tags is recorded in the system. (For example, merging changes also the tags displayed in the revision history. For example, you can look at this question: Is there any progress toward solving Gilbreath's conjecture? The revision history shows open-problems, despite the fact that when the tag was added, it was called open-problem.)
The description of the history of the tag is based on the past discussions: Alexander Chervov's post on meta and a related discussion in chat. You can find a bit on the history of this tag also in this question: What should the tag (open-problems) be used for?