From memory it will fly anywhere between 15 knots below programmed DES SPD to VMO -16kts and stay in PTH, outside that it'll either add thrust (SPD-PTH) or it'll say drag required prior to intervening itself and dropping from PTH to VNAV SPD as you said.
The -8 displays this band which makes me think drag required doesn't appear till the top limit is reached likewise 15kts slow nothing happens but at 16kts SPD activates.