We activated the PageSpeed Nginx web optimization at all default accounts
This optimizes CSS / images / javascript. Our tests have shown GTmetrix by site multiple improvements ranging from 10 to 25% ...
Normally no problem with the sites but not impossible with CSS modifications, /ex, Pagespeed uses a cache system which purge itself every 3-6 hours.
Also do well CTRL + F5 et SHIFT+CAPSLOCK+R
You can disable PageSpeed temporarily or permanently.
If you want to desactivate PageSpeed, you must change your profile Nginx :
https://support.yoorshop.hosting/knowledgebase/1891/How-to-enable-or-disable-Page-Speed.html