Apparently, there is a section on the right side of the screen which Dodongos will not enter. If they spawn there, then when they leave, they won't go there again.
To show this more effectively, I used a script to spawn 100 of them.
[Beta 18] Dodongo AI
Fixed. Looked deliberate, but I don't see why this would be desirable.
Improved, though I've noticed that dodongos still avoid the 2 rightmost columns on the screen, but will go all the way left, up, or down (with the right quest rule).
Only the single rightmost column, from what I'm seeing. That's related to how they walk; when they turn from horizontal to vertical, their new position is always where the left half was. That's how that are on the NES, so it won't be changed without at least an attribute to control it.
Oh, my mistake then. I did not know about that quirk. I also found out why they weren't walking on the 2nd column from the right in my test quest (the bushes on the right). That being the case, I think the bug can be considered fixed.