Are you sure? Previously I've heard that it applies the lock whereas Choice Band doesn't, which is what tipped us off to the possibility that Choice Band was disabled during Dynamax.
Yes, I am sure. I am on a phone, but you can watch shofu's battle spot matches from twitch on 11/17. In it he uses Darm, a lot. One battle he dynamaxed immediately with scarf and was able to freely choose moves and was then allowed to choose his move after dynamax and remained locked.
Another battle he locked into ice punch, dynamaxed to use max quake I think. After dynamax he was locked into ice punch again.