Guests are no longer able to access SharePoint site

Previously we required guests that receive an invitation to our SharePoint site to create a free Microsoft account and then accept the invitation. Recently we have found folks that have a Google account are able to accept the invitation and can access our site with a one-time passcode. Those that are doing that, are losing access periodically and it is become a nightmare to assess. They receive a message that "We couldn't find your account" or no account exists. We see them in our AD as a guest, they are permissioned to the SharePoint site, but continue to get an error. Only when we remove them entirely from our tenant, have them create a free Microsoft account and then accept the invitation will they have consistent access without errors.

Did Microsoft make a change lately that is causing this disruption? Any ideas on how we can mitigate it?