That's the rub: clearance. If a vehicle starts to change lanes but then has to abort & return to their original spot, another driver anticipating the vacancy & proactively moving to occupy that (not yet vacant) spot has failed to leave proper clearance.
It really doesn't matter if either is a motorcycle or an autonomous vehicle, it only matters that the driver that did the anticipate/proactive bit failed to give the proper clearance for safe driving. The autonomous one probably would never do such a thing (it would detect that the lane isn't empty yet & thus refuse to move into it) but cyclists (filter/split) it all the time.
As my defensive driving instructors always drilled into my head, "You & your bike might weigh a thousand pounds. That car alone weighs a ton or more. Who do you think wins in that contest? Here's a hint: it. ain't. you."
I've often done the filter/split maneuver, but *only when I'm sure it's safe to do so*. Attempting it while the car I'm about to drive beside hasn't finished lane changes is outright suicidal; I don't know if the driver will change their mind, swerve back in, & turn me into a pap smear.
Yes a Human driver aborting the maneuver *might* look back to make sure their lane is clear for them to abort into, but they shouldn't have to. If they haven't completely left it yet then any other driver trying to occupy that same space would be guilty of trying to sideswipe me.
I agree you need to look out for number 1, but if you think that was a good time to try & pass another driver then you'll be reduced to a bunch of number 2.