Unsubscribe fails but appears to have worked for resending to previous mailing to search results

Recreate with 5.2.2 (on Dupal 7)

1. create mailing group A
2. create other group B
3. add contact into group: B
4. do mailing to contacts in group B, using group A as unsubscribe. (i.e. Find » In Group B » Action: create CiviMail mailing)
5. Create and send mailing: sent to recipients of mlg1
6. click unsubscribe.

Expect

to see confirm form saying "you will be unsubscribed from A" because A was the base group used for the original mailing. (When sending to previous mailing recipients there is no option to select an unsubscribe group)

Actual

User sees "yes you've been unsubscribed from the mailing group/list" code ref but nothing has actually happened!