Hi again,
this is something not expected of course. We have no other cases reported.
When the parameter below 'Match existing user' is enabled, if a user login through Facebook using an email address already registered in Joomla, a random password is generated to allow the social login on the fly but the original Joomla password is not altered.
If this happens on your website i've no clue about the reason, eventually you can disable the parameter 'Match existing user'.

Thanks,
John