It's not that. The one thing I hate about AP right now is that at every onramp it'll slide half out of the lane and then abruptly back in as the merge happens, it sees itself in one big lane and then the lane narrows.
Some states like Tennessee have dots on most of the onramps which stop it, but otherwise I have to choose between an uncomfortable joggle that looks drunk/distracted or breaking Autopilot for every onramp. (Or driving in another lane, when there's no proper reason to.)
That is the only issue I've had. Fortunately, I'm rarely in the right lane, usually just keeping up with the flow in the left and occasionally moving over for people less concerned about a speeding ticket than me in which case I just move over, let them by, and move back.
One feature that Tesla could add to solve this would be to give AP the option of Left Bias / Centered / Right Bias. That way if you chose Left Bias it would keep a constant distance from the left line when the lane widens. In the UK you could set it to Right Bias.
Just a thought. Probably other solutions as well.