I placed this in Bug Reports instead of Feature Requests, because I think users generally should be getting notifications for flags they created and commented in (which OP did in the example flag posted). This should automatically subscribe them to the flag. I am not 100% sure how the system works though, so happy to hear more details on this!
There are other bug reports for flags which this might be related to:
https://forum.inaturalist.org/t/no-notification-of-resolved-flags/29974
https://forum.inaturalist.org/t/not-all-notifications-received-for-flags/31206
https://forum.inaturalist.org/t/didnt-get-notifications-of-comments-on-a-flag/39593
They all seem similar issues with idiosyncratically not receiving flag notifications that people should.