Immediately after updating to macOS Sequoia 15.7.8, Google accounts stopped authenticating in Apple Mail. Internet Accounts shows Mail enabled, but the accounts do not appear in Connection Doctor or remain inactive in Mail. Removing and re-adding the accounts does not resolve it. Other users are reporting the same behavior after 15.7.8.
MacOS Sequoia 15.7.8 is a security-focused update release, not a beta, as far as I am aware. However, if the discussion forums do not wish to accept it, we can escalate the issue to the Mail team by submitting a bug report. Please let me know if you would like to pursue this route.
If you have any questions about filing a bug report, take a look at Bug Reporting: How and Why?
If you decide to go that path, I'm happy to help and I'll route the bug straight to that team.
Once you open the bug report, please post the FB number here for my reference.
Albert WWDR