If it was a single lane, then technically the bus was attempting to pass a moving vehicle in the same lane as itself, which puts it at fault.
So either the AV broke the law first (by doubling up in a lane), or the bus had the right of way. I'm with the other commenter, why isn't it just an actual turn lane?
Also, if the bus is at fault here, then this would be an amazing place to commit insurance fraud. Just sit in the right hand part of the lane, wait for traffic to pass by, then drive back into the left hand portion of the lane right as a BMW/Mercedes/Ferrari/etc drives by.
The AV can proceed "when safe" to approach the intersection and then, "when safe", make its turn, but the bus owns the lane otherwise.