I have completed almost all achievements in game. The counter on my alliance character says 3707 / 3717, that means 10 achievements left. But in reality I can find only 7 of them, 3 “other” are not available legacy “Brawlers guild” season 3 achievements, they still give AP (I would show you but cant add screenshots)
I asked the GM to give me a credit for those 3 achievements but he sent me to wowhead lol.
In most cases GMs can’t give you credit for an achievement.
Blizzard won’t give you them, even if they will complete your collection.
Can confirm that just granting the achievements to your character will not be something Game Masters can do. Best thing to report bugs is to use the ingame bug reporting tool as those go straight to the developers. There won’t be a response with that report but I can assure you that they do get read.
To submit a bug report, open the game menu by pressing esc on your keyboard and then click the help button. You should see the bug report option on the left of the window that opens. 
But it’s not a bug, it sounds a bit like OCD.
Well it does seem rather odd that old Brawlers guild achievements are still registering as available to get in the system as we are now on Season 4 and not 3. Perhaps should have been clearer that this is what should be reported as a bug though!