Having the same problem with one of our sites using Full SSL (not strict). From my testing, it shows up in Chrome mobile on an android phone, but not on my desktop Chrome, guessing it’s due to version changes and how they’re handling ECH differently.
Temporarily pausing cloudflare on this site (bypassing and using the original server IP address) seems to resolve the issue as a workaround, but is obviously not a permanent fix.
I see that ECH just started getting rolled out by Cloudflare over the last week, so I would also like an option to disable and opt-in later once the bugs have been worked out with Chrome’s handling of ECH or whatever is going on.