I can only guess, but I believe it’s working correctly both seamless and not. It seems that the tracking status is set earlier with Seamless Play so the page is already completed before the action to increment the variable is run, i.e. is not run due to the condition.
If you turn the logic around and count the completed pages it’ll work.
As I wrote before, it would be easier to handle the tracking status if a page was set to “In progress” On: show (if not yet completed) and to “Completed” On: Hide.