xenforo really works well with all these php releases.
There is a lot that can be wrong with Multiphp, maybe you need to restart php, maybe restart php-fpm, there might be a problem with your webserver (apache or nginx) and its configuration files.
My advice is to upgrade to php 8. If there are issues, at least you upgraded to the latest php. Multiphp is not worth it if it can not provide the latest php 8.