Cannot change mathtex.cgi location

Before updating to WordPress 5.xxx and getting the latest version of your plugin, everything worked just fine. I have a custom location for mathtex.cgi (cgi-bin folder of my apache2 name-based virtual host).

Going to that url directly https:/dummy/cgi-bin/mathtex.cgi?…. still works !

I’ve tried to set the mathtex.cgi location using the settings page of your plugin, but my changes are ignored. Upon reloading the page, the previous (wrong) entry appears again.