I’ve caught several Awoken Coelacanth on Saturday for the fishing derby achievement. None have counted.
You have to have the hour long Derby Buff active for it to count.
If you’ve already turned in the quest then you’re already out of luck. The Achievement actually says you have to have caught said fish while doing the Fishing Derby quest and having turned it in on time.
Good news is it is shared with every other week of the event so you can do it next week and try and get the fish you missed.