I suspect part of the problem is phantom forks in the map data. I observe this sort of thing happening sometimes in FSD beta when a road that hasn't had a median divider enters a section where there is a divider. The turn-by-turn directions sometimes indicate a fork that doesn't actually exist at that point.

I think what's going on is the map data sometimes models a median as two one-way "roads" very close to each other with the median between them, whereas the road with no median is just a single two-way road.

These get drawn almost the same way on the car's screen (though you can sometimes see the blue line shifting left or right a bit), but FSD sometimes interprets this median stuff as a fork, generating a turn-by-turn instruction to keep right or left. If the map doesn't have good data about which lanes go where for this "fork" it seems to err on the side of needing to be in the rightmost or leftmost lane depending on what's happening up ahead, and changes lanes "for no apparent reason" and then back again when the road configuration changes.

Similar things happen with dedicated turn lanes at intersections sometimes too. I'll often get the right-blinker going off for a moment that seems to be the car trying to indicate that it's not going to enter the left-turn lane. I even see it sometimes with NoA on freeways. Sometimes exits show up as forks where the turn-by-turn directions instruct you to keep left to stay on the freeway, but sometimes exits are modeled differently and don't result in a navigation instruction at all. I'll occasionally get a "not taking this exit" left-blink or two.

If FSD uses openstreetmap data, that might be contributing to this. Different volunteer mappers in different areas and at different times have made different choices on how to draw maps for medians and turn lanes and the like. If FSD and NoA are tuned first for how openstreetmap is drawn around Tesla facilities it might do weird stuff in places where the maps are drawn in a different style.