As it turns out, if an Unseen Saboteur causes the Baited Arrow hunter spell to be cast on a target where the spell, if cast normally, would overkill, the overkill does not trigger to summon the expected 5/5 Beast. This may be a more general interaction bug between the Overkill mechanic and Unseen Saboteur, or something more specific limited to Baited Arrow or a smaller subset of spells.
Working correctly.
Overkill - Deal excess damage on your turn for a bonus. (bold emphasis mine).