Hi Tim,
I think this is what is happening:
When I look at the very first second of your video, I can see that you are in a peculiar protection situation - the barber pole (max speed) is still at 230 kts, which indicates that you JUST retracted the flaps, and most likely got into an overspeed situation where the autothrottle reversion mode is trying to prevent a flap placard bust. Hence the MCP SPD both in the autothrottle and pitch channel FMA.
Once the flaps fully retract, the overspeed situation is rectified and the plane reverts to VNAV SPD climb. Why it does that I don´t know, it should actually go back to FL CHG climb.
I would dare to say that the protection situation (reversion modes) is a bit of a black hole, and the flight manual isn´t very clear about those either. I don´t have much (any) experience with those in the real plane or simulator (I am proud to say). We have coded it to act like described in the flight manual, but it isn´t very clear on what happens when you exit the overspeed situation. I see that the plane goes back to MCP SPD and CWS P when you push the VNAV button, as expected.
Try to fly the departure again and make sure that you don´t bust the placard speeds and see if it still happens.
Cheers, Jan