This is related to Stack Exchange is intermittently failing at random areas and, possibly, The list of related questions is shown empty, although the latter one involved all questions and there was at least an empty "Related" header showing. Questions on this site posted before Search on MSE is broken have related items showing, but I've confirmed none of the new questions since then do, including even having a "Related" header. I've also noticed that recent Math SE questions exhibit this same behavior, so the problem appears to be network wide.
The discussion among the comments of Stack Exchange is intermittently failing at random areas indicated the issue was related to searching, with a link to Nick Craver's tweet provided indicating that issue had been fixed. Nonetheless, the timing of having related items no longer being shown (and likely not even being determined) for any new questions at the same time strongly indicates to me this was also an issue that occurred then but which has not yet been fixed.
Update: As Sonic the Masked Werehog's comment stated, this appears to be a regression of a much earlier bug described in "Related questions" section not showing up on new questions.