In the last phase of the rendezvous autopilot, it sets up maneuver nodes to "get closer," and then again to cancel all relative velocity. However for that very last maneuver node (canceling rvel), it does not use timewarp to get to that maneuver node. Instead, it just sits there waiting for the burn.

In my case, the maneuver node is usually about a minute out, so using timewarp should be possible.

It also looks like this is disabling auto-warp. Activating auto-warp in the ascent AP for example does not help, it will be deactivated again automatically.