There's no point into change AI core routines because they have no room to improve until SCS could leverage way more CPU for them.
What they had do for sure, more than once in past times, is to touch some parameters. I guess values about the distance in where the AI considers other vehicle is too close to don't merge. 1.39 had a big change respect to that for sure. They probably keep touching values of that kind testing. And of course, any of that kind of changes is going to cause havoc with high density traffic mods.
If you add to this the usual amount of bugs in prefabs on a new DLC, where yield zones aren't well defined; the cocktail is explosive.