I experienced this as well, though in the end only a handful of people had mailing list mode turned on at all and were affected. I was able to write to them directly to sort it out. So it was a fairly easy transition once the settings bug was fixed. Unfortunately one of those people was my CEO who was not especially happy about it.
To find out who had mailing list mode turned on and the frequency, I used this data explorer query and then exported to csv.
SELECT user_id,mailing_list_mode,mailing_list_mode_frequency from user_options