What do we need to do to qualify for this medal?
To qualify for that medal, you'll need to complete the Second Sunday event. For February and March, that means complete 6 different missions on Sunday from 00:00 to 23:59 local time. Niantic might change what is needed in the future, but for now, it's missions.
No longer “unique” missions?
just noting the change in language. Is it truly just ANY 6 different missions?
You're asking about a rules change that only exists in your head. "Unique" and "any" don't mean anything different in this context. Basically, it's been "any 6 different missions" since the the second Second Sunday.
My husband and I completed 6 missions on March 12 and did not receive credit for Second Sunday perticipation. How can this be corrected?