I got two more stuck role sims, one right after the other. OW took me to the same spot again. This time, I backed off and did my best to see into the camera unroutable space, and this is what I found.
https://i.imgur.com/HUOa4f4.jpg
The sims unroutable paint on that road now starts at to the right of that last building. The camera unroutable paint starts in the middle of that first curve where there is no rock paint between the road and the mountain. That's where OW always takes me when they're stuck out there. But there's a lot of actual map past that point, before it gets to the distant terrain. I'll check the terrain paint out there, but if I can't find a place that's missing it, I'm going to erase all the unroutable paint on that road, and any surrounding terrain that isn't too steep for sims to walk on. Hopefully, that will take care of the problem.