Three related bugs (?) here. I don't know the exact conditions when it happens, I have only examples.
a) Situation: Admin changes User groups (Secondary) of another user in the ACP.
Problem: In some cases, the settings who can make conversations with this user, and/or profile posts at it's profile, change too, eg. from everyone to member-only. It's visible both in the form (if called again after saving) and the user change log.
b) Same situation (Admin changes User groups (Secondary) of another user in the ACP).
Problem: In some cases, the user change log shows that the users signature was edited too, altough the old and new value are the same.
c) In the user change log, I have quite a few entries from users (editing their own settings) disabling the mails for conversations, and enabling it at the same time (ie. two blocks of changes at the same time, one from Yes to No and one from No to Yes).