The site was switched HTTP/3 but none of online checkers show that. Google console not showing neither. I purged cache - no help. How to make sure it works?
- see if there are HTTP/3 requests from the Analytics page, https://dash.cloudflare.com/?to=/:account/:zone/analytics/performance
- use an HTTP/3 compatible browser and check how it connects. Note that some require a toggle in the settings.