I have my sites on Cloudflare in the past with this same issue then never thought of it then moved to another hosting service that doesn’t need CF, it had its own inbuilt CDN and so on.
Moved to another hosting and added my sites to cloudflare again and the not mobile friendly issues started again, actually on paper, at a glance, with the human eyes and other tools, the pages are mobile-friendly but the tester in Search Console doesn’t always think so.
To actually diagnose and see that CF is the cause of this, I deactivated CF on one of my sites and within a day, all the errors are gone in search console. I think CF bot is not letting a CSS load properly or let a crawling bot through.
Anyone facing this same issue? Any help? Very appreciated.