I revoked the authenticator and uninstalled/reinstalled the app. This was done with both "OEM unlock" and "Developer options" disabled. The app opens and shows the welcome screen without errors.
Then I enabled "developer options" and "OEM unlock", and rebooted. App still opens without showing any error. Then I uninstalled the app and reinstalled once more, to make sure I'm forcing a "re-validation" of the device. Welcome screen still shows without any error.
Now I need to activate (enroll) the authenticator. I started the process but for security reasons there's a waiting period for that (*) so I need to wait for it to elapse. I will try enrolling the device in the current state, with "OEM unlock" enabled, and see if it works.
Will keep you posted.
(*) They send the person an SMS notifying that there's a request to enroll a new authenticator, and you have time to contact them and block it if it was not you who is doing it.
P.S.: In the MitID website they provide an activity log, and I checked what happened yesterday, and there's the following entry: "App - temporarily blocked due to 'jailbreak' - unlocks automatically once it is removed". This confirms that something tripped the app and it reported to the mothership; but the wording claims that it would work again once the right conditions are satisfied. This clearly did not happen for me. Another interesting fact is that yesterday, at that point in time when I launched the app and it locked itself, I already had all the other GmsCompat+Google updates that other reporters claimed that "un-broke" the app.