We've had my son in a Montessori school for 3 years and are switching to the public school. As you well know, Montessori has good and bad points. You kind of "do your own thing" as deeply as you want.
That is great but when it's time to do something boring or something you aren't good at (boring math problems, etc), I think that Montessori kids may have trouble dealing with that. Maybe some of what you are seeing is that.
I would guess that really ADHD kids may not be able to pull themselves together as quickly as your son did. Maybe he is bored and a little disorganized, like so many 10 year olds are at this age.
I know in the psychology literature, if you pay people to do something, they do it, but in the end, they are less committed to doing the task than if they internalize the drive to do it.