Specifically the following quests will become unavailable if the new fishing skill is already learned:

https://www.wowhead.com/quest=72252 (Alliance)
https://www.wowhead.com/quest=72253 (Horde)

https://www.wowhead.com/spell=366253 (Dragon Isles Fishing skill spell)

(It is likely this is also the case for all other profession quests, it just happened that I missed this one on release night and had to go back and do it on an alt)