Date & Time: 04/13/2021
Frequency: Reported by several people
World name: Cerberus
Character name: Schwarzwaelder Torte
Party or solo: Both
Steps:
1. Join an Ocean Fishing voyage
2. Encounter dolphins or seagulls
3. No visible status is triggered ("Cetaceous Speed"/"Auroral Flipper"/"Gullstorm")
4. No visible progress in the achievement menu under "Mine, Miine, Mine" and "Double-backwards Somersault" (both stay at 0/3)
5. No bonus at the end of the voyage for encountering seagulls or dolphins
6. No visible progress in the achievement menu under "Mine, Miine, Mine" and "Double-backwards Somersault" at the end of the voyage (both stay at 0/3)

I was alone on a boat, saw 2 dolphins in zone 1, 1 dolphin and 1 seagull in zone 2, and 2 seagulls in zone 3. Other people I've talked to have reported not seeing any progress on their side either, be it with 1 seagull/dolphin during the whole voyage, or seeing 3 seagulls on the same voyage, solo or in a party.

It's also possible we just might not have figured out how this new feature works but.. based on the achievement description alone, it does look like a bug so far.

Screenshots to prove I did encounter 3 dolphins and 3 seagulls on the same voyage: