Biters are unable to pathfind through my chokepoint and just give up

[NuclearFuel]

I set up my base in the middle of a lake with a very twisty chokepoint to keep the biters at bay easier and to make the most use of my flamethrower turrets. However, despite me being able to drive a tank through it with relative ease, the biters completely give up before they even start biting at the gates. Here is the chokepoint:

(As you can see I've been playing around with the map editor)
The biters usually start braking right before the gates as they run to them. Those unfortunate ones that do not brake fast enough gets caught in my flamethrowers. Once they have reached a halt, in 95% of the cases they stand still and do not move for like 5 minutes (without even attacking the gates), and in the other 5% they scatter and run away. I don't get why this is happening, since they should have the capability to walk on 1-wide tiles. Does anyone know?

[DerGraue]

It's called "superior biter AI". They go there, see the defenses and think: "Thanks, but no, thanks." And then they walk away.