I haven’t experienced that issue for now, either as Google says it is possible to opt out of HTTP/2 crawling. The server must be configured to send a 421 server response code.
Maybe Cloudflare needs to update their “known bots” to make the “new Googlebot HTTP/2” as a “good one”, confirm and verify the IP addresses?, and allow the requests to pass?
Have you tried contacting Cloudflare support regarding this issue?
As stated due to crawl error troubleshooting:
If the above troubleshooting steps do not resolve your crawl errors, follow the steps below to export crawler errors as a .csv file from your Google Webmaster Tools Dashboard. Include this .csv file when contacting Cloudflare Support.
- Log in to your Google Webmaster Tools account and navigate to the Health section of the affected domain.
- Click Crawl Errors in the left hand navigation.
- Click Download to export the list of errors as a .csv file.
- Provide the downloaded .csv file to Cloudflare support.