Creating this ticket for the record.
The s7 instance was totally stuck and the instance was delayed 14h on this query:
| 17 | system user | | fawiki | Connect | 53781 | updating | DELETE /* FlaggedRevs::clearStableOnlyDeps */ FROM `flaggedrevs_tracking` WHERE ftr_from = '0' | 0.000 |
I was not even able to stop the slave.
I had to kill -9 the process.
Converting the table to InnoDB fixed the issue.