Account changes in hook_openid_connect_pre_authorize are not linked to the user via External Authentication (externalauth)'s linkExistingAccount / authmap table. This also has ramifications on the logout, because if the user is not linked via externalauth the logout functionality does not fire - i.e. the user will not be logged out of their IdP even if you specify that they should.
In OpenIDConnect->completeAuthorization, if the $account is a UserInterface, link the account before saving the user info.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Would this issue also cause a change in a users sub to not update in the authmap table?
I'm not sure, but it certainly seems plausible. I've had this patched locally for a couple years so I'm not positive, but that does seem vaguely familiar and might be one of the things I ran into. I'm planning to open a merge request with the changes.