For demonstration, I have the following setup, in a brand new entirely-vanilla sandbox game, playing from the editor:
A single gun turret, with a thick, beefy wall on one side, and a weak wall of pipes on the others.
Now, as expected, the biters will pathfind around the difficult-to-breach wall in favor of attacking the much weaker pipes to get to the turret instead. This makes sense, and is exactly the behavior I would expect. (I am using the "show-paths" debug option to display the biters' paths)
But watch what happens as I move this setup closer to the chunk border.
With one tile of space, it once again works as expected, the biters pathfind around the wall just fine.
But once the wall touches the chunk border, while some biters still go around, some biters decide to just go *through* the wall instead. There is no reason for them to waste time chewing through a tough, hardy wall when it would be much faster to just run around instead like their friends do.
This behavior continues as I move the wall through the chunk border:
And it isn't until the wall is all the way through the chunk border that the biters will actually consistently start going *around* it again.
I have noticed this sort of "aggressive towards things at the chunk border" many times while trying to design effective walls. If I don't make sure it's spaced out from the edge of the chunk, they'll eventually destroy my obstacles every time, rather than nicely walking around. For a practical setup, here's my starter iron mine design I've been futzing around with (chunk borders displayed with the ChunkyChunks mod, the green circles are from the Bottleneck mod):
If I spawn a horde of biters to attack from the Southwest, they nicely funnel through the pipe maze, giving the turrets ample time to shoot them all. After the attack, there's rarely a single scratch on any of the pipes.
But when I spawn some biters to attack from the [radially-symmetric] Northeast, they go on a devestating rampage and chew right through the pipes every time.
It's the exact same pipe maze, the entire base is just two identical rotated copies of the same blueprint, but these behaviors at chunk borders mean that it's dependent on where exactly the resource patch lies, and thus where the chunk borders fall in the pipe maze, as to whether or not it is effective or just a huge waste of resources and time.
I've included my log, as well as the vanilla demonstration save (biter_pathfinding.zip) and my slightly-modded deathworld mine testing save (walltesting-deathworld-setup.zip).