I haven’t tried it recently, but previously I found you could still fish after the event was won you would just end up with ‘empty’ pools most of the time because the tastyfish were hidden/gone. However, if you caught a rare fish you would still successfully loot that during the stated event time.
I tried to do this today and it didn’t pop up on Lightning’s Blade.
Tried fishing a bunch of pools anyhow in hopes that the item would activate the quest…nothing.
Guess I’ll try again another day, maybe will all Add-ons disabled but I dont use many so idk - very disheartening.
Can confirm that this is still bugged on Gurubashi (NA). Got 39/40 fish at 2:15, and the NPC suddenly announced that the tournament was over. The fish vanished from my inventory, the WQ vanished from my quest log, and the excitement vanished from my face.
It is 2023, and I just had the same thing happen to me.
I can’t believe this is still an issue six years later.
I’d be fine with not winning because, hey, I didn’t expect to. But I can’t even turn in my excess fish. They all disappeared as soon as the messages fired off back-to-back at 15 (and 16) past, respectively, that the tournament was halfway over, and then over.
Again, don’t mind losing. But the fact that all my tastyfish got the General Zod treatment is really frustrating.
maybe its because its not a huge concern for Actiblizz. maybe they’re being told its “working as intended” and that those griping about it are “sore losers”. personally i won the competition many years ago and snagged the heirloom ring. i wasn’t even expecting to win, but i did. it was also the last achievement i needed for the Salty title…which i typically use on all my characters from that point on.
i have zero intention of ever going through that nightmare ever again, so even on Classic that title will forever elude me, as will many other holiday achievements that require PvP. that being said, it might be wise to not necro this thread any more. if they haven’t addressed the issue back in 2017, its not likely they’ll address it roughly 7 years later. 
Doing it today, the event popped up for a split second, then disappeared. I fish pools and dont get any tasty fish just the basic ones. I turned off all addon’s , still nothing. Relogged with the addon’s still off. Still doesn’t work. Annoying waste of time. I’m on a dead server too so it would of been my first easy win. Also tried it in warmode and nothing changed. WQ isn’t showing.