Ambling Caltrop...

[Fruity_Snacks]

I wouldn't expect them to 'fix' any older fish. AFAIK, they have never nerfed a fish via code adjustments.

At the start of Stormblood, various big and legendary fish in ARR/HW were updated to have higher bite rates. Opabinia, for example, was one of those fish.

This is a unique situation according to OP, which would require a unique solution. If they've never broken a fish like this before then of course they've never had to fix it before.

This implies that fishing with the quest done/active means the fish is "broken." Broken is crashing a server when it's caught (King of the Spring, Patch 3.5). Or not being able to trigger intuition (Aquamaton, Patch 6.2ish). Or being able to catch a fish with a versatile lure straight-catch when it should only be a mooch (Basilosaurus Rex, Patch 6.4, Chlorophos Deathworm, Circuit Tilapia, Durdina Fish, Patch 6.5).

Based on what we know so far about big and legendary fish in Studium/Facet holes is that they are designed, assuming that you have the quest done. If you do the quest, then you are fishing for the fish as is intended. The fish bite rates in these holes ARE changed and rebalanced based on if you have done the turn in quests unlocked or not.

Does that make it feel any better? No. But from what we know (again, so far, we're still working to study this), SE is aware that this is a thing and does modify the fishing hole around to compensate.

...and this isn't supposed to be one of the ultimate rare fish so it was clearly an oversight IMO (it's a 5.2 fish)

While typically early patch fish *have* typically be 'easier' it's usually meaning they are up more often. This is not always the case though and is not completely the norm, see: Cosmic Haze (6.3), Priest of Yx'Lokwa (5.1), The Second One (3.3), Aphotic Pirarucu (3.3), The Matriarch (2.3, OG was rough), and then even common fish like Hanatatsu and Saltmill in 4.0. Patch is usually a decent indicator of 'how much time will you spend on the fish' but it is not, if ever, perfect.