Beats me. It's a very plain Azure VM made just to run Moodle, made from a stock CentOS 7.4-minimal image from Azure Store; Apache, PHP and the database installed via yum, PHP has just the extensions to satisfy the Moodle requirements, and when stuff started to act up I disabled everything (SELinux, mod_security, mod_evasive) that colud get somehow in the way.
More, the same thing is happening with Moodle 3.5. Folks here got a sudden passion for being GDPR compliant, so I had to install a 3.5 and in the middle of the setup the site lost all styling, so I had to follow the installation steps in raw HTML. Once installed I tried to make sense of all this, but without luck: all the stylesheets the page requested were being served with a HTTP 200, the correct MIME and actual CSS inside, just no style was applied to the page. I gave up and turned to what I knew works: switch to the Clean theme, patch that one cached file in localcache, refresh and suddenly all is well...