Yup, this was a bug that was introduced to the query yesterday and a fix is rolling out shortly.
We've been dealing with a lot of query performance issues since we upgraded to SQL Server 2019 at the end of March. The query to populate the First Post Review Queue has been a nagging problem on some of our sites (mainly Math Stack Exchange). After poking many different things to get the query to just work faster, we rewrote it, and unfortunately screwed up the logic. We rewrote the logic again this morning and the queues should start to populate again.
Your observation is correct, though incomplete ...
On ALL sites, and with a few exceptions, no posts are being added to the First Posts review queues.
This appears to be the case since yesterday afternoon / early evening (CET).
There seems to be a few exceptions, such as on SO, where there are currently only 4 (FOUR !!!) completed reviews. And for one of those reviews it appears to be an audit review (so for those sites that have audit reviews enabled, those kinds of reviews still show up).
Note: Late Answers reviews appear to not be impacted.