DKR, Good eye for detail.
If the plate material is green it will shrink. Being pegged to the tenon in both places may seperate the two halves of the scarf as the plate drys.
Drawboring the pegs will keep the lower half tight to the post but may not be enough for the top half. The pegs in the scarf and the through tenon should be enough to keep it together.
I have also seen in old frames the through tenon cut just shy of flush with the top of the plate. Presumably so that it would not stick up as the plate shrinks. Tom