Metaphor: Imagine weights as rocks in the path of the pawn.
Preferable has no rocks
Normal has a few pebbles
From there it goes from loose rocks to large boulders
(More in depth around the apparent problem)
If maps that have tons of "avoid-path weight from this mod" or real rocks/trees between the pawn and their destination the vanilla pathfinding seems to have a kill switch which tells the pawn to make a straight path from the last square the pathfinding was working on. More specifically
(Even more in-depth)
if i'm reading the code correctly - which i will not guarantee

- it seems that on the eight square, if the path finding is still stuck with a bad route it will kick out of the loop and just draw a straight line to the destination from the eight square
(Summary w/ some extra in-depth stuff)
I think the main root to many people if this mod's "normal" having a weight other than 0. This is done because in the background the weight values can only be from 0 to 255. So for preferable to exist, normal has to have a non-zero value. I'm thinking there are cases where the game's path finding is even counting relatively low difficulties for travel as non-optimal and going through the kill switch.
(Possible fix for those still having problems)
For anyone having a problem they could try setting the weights as Preferable = 0, Normal = 0, and the rest the same. Make a new map or load a save and verify all squares are either Preferable or Normal. See if the pathing is improved at all.