I wanted to share some of my observations and tests about fishes getting away.
I've noticed that most of the time, those fishes have a strange physical behavior in the water.
You are all right, it's always (99% of the time) nice fishes, trophy or not.
I've experienced this strange behavior on many different species: carps, sturgeon, common bream, white bream, pike, etc...
What I've noticed: those fishes are strangely difficult to reel in, even for small species. They seem to oppose a bigger resistance to water, like they are stuck between 2 layers of jello.
The tension on the line stays pretty "normal", but the fish stays pretty immobile. When you do nothing at this moment, just keeping tension on the line by staying with the rod raised, the fish does not move: no rush, no lateral move, it just stays at the same place, fidgeting.
You may encounter this kind of behavior in plain water, without obstacles, but you can experience it when the fish is in reeds or lilies too.
When it happens, I know 100% that this fish presents a high risk of getting away.
So I change my fighting style when I encounter those fishes: I try to make move slowly the fish without reeling, just applying tension by pulling on the rod, without right click, just pulling it by mouse movement. Then I reel a bit to get less line while lowering the rod, and I pull again by mouse movement. You will observe that the fish will move with difficulty. If you feel some strange resistance, don't insist. The fish will finish by unjam and do a rush or just finally normally move. But you'll have to be careful, as the fish will probably have this behavior again.
To sum up: no reeling on the fish to make it advance towards you, no right click to raise the rod, being careful.
I've encountered a fish like this yesterday, I tried to fight it normally to confirm my observations, and it got away!
I usually fight those fishes the way I described since 2/3 months, and I can tell that it works pretty good. I think I've reduced not bad my ratio of fishes getting away.
But I still encounter some very random things, like fish is running fast and gets away for no reason (nothing that could have been noticed).