Feb 17 2021
I occasionally get this issue on my wiki as well. The "fix" I use is to add a space at the end of the latex and save the page, which usually makes it re-render properly.
Nov 24 2020
I just wanted to say that when I downloaded version 1.35 of mediawiki on a debian 10 machine (2 days ago) , it still had this problem, the solution provided where one adds AllowEncodedSlashes NoDecode indeed worked, and for anyone who is not sure how to do this, one must ssh into the host machine, then edit /etc/apache2/sites-enabled/000-default.conf, add that line, and then restart apache.