It can't be fixed because the wowace forums and the curseforge forums are really the same forums using the same backend database. Just differently skinned.
This is not 100% true (maybe 98%). I am subscribed to 7 forums. 3 of them are listed in the WoWAce/UserCP while the other 4 are listed in the CurseForge/UserCP. Both versions of all 7 forums show me as being subscribed, yet they don't all show up on both versions of the forums.