In Multilogin UPDATE mode status "OK" is sent only if all the sent tokens are valid. Otherwise the status is INVALID_GAIA_CREDENTIALS and Multilogin is not successful.
In PRESERVE mode it is fine to have failed accounts so the status code in this case should be "OK", but the tokens for failed accounts still have to be invalidated.

Needs discussing and confirmation with Gaia engineers.

See oauth_multilogin_result.cc