What I found in Florida is the double HOV lines are spaced about two feet apart and that's what the car is objecting to. There's enough room for a motorcycle, and I've always wondered if that's the reason.
The car thinks you are trying to get it to go into a "lane" that's too narrow for it, so it won't make the lane change. Its actually a side-effect of a safety feature preventing an unsafe lane change.
Are you seeing different behavior? If this is the problem, you then know how to handle it: drop out and then back into AutoSteer when going across these weirdly spaced lines.
Just one of the many, many special cases which need to be handled eventually for FSD to become real..