I play on World with many islands, and biter attack groups just... forget midway to my base that they were going to attack it. Either this, or they just stand near their base indefinitely, like they couldn't pathfind.

This mod does not affect pathfinding of biters at all. The only thing it does is allow them to traverse water which they should see the same as land.

I think you should look at your other mods.

Then it's probably something with Rampant. I've just checked, and saw that it can happen with it :/

Ah with rampant the biters do tend to collect a larger group closer to your base which with swimming can be in the water. At some point the entire group should attack your base. Also if a base isn't spawning more biters due to lack of players close by or pollution the biters may wait a very long time before attacking.

Nah, it's not that, they don't attack at all, like, in the last 10 hours of gameplay, and with lots of pollution they haven't attacked even once. I've created a new thread on the Rampant post: https://mods.factorio.com/mod/Rampant/discussion/64442874234b4b325fa93a42
We'll see how it goes.

I'd guess the same happens on land too since there isn't really a difference between water and land with my swimming mod for biters.