Cannot mark answer as solution after some time

I noticed it before but just remembered now:

I couldn’t mark an anwer as a solution in this thread:

(I just don’t see any “solution” checkbox at all) but in another thread it worked fine. The only thing I can think of is that I didn’t mark it as solved right away when viewing Joaquim’s answer, and when I came back to it a few days later it was not possible.

A bit weird, but maybe that’s just how the discourse plugin works…