8 speed 8th gear problem

[silveradosid]

2020 silverado 2.7 8 speed 118k miles. using m7 the transmission shifts fine up or down, if i just put it into drive the 3-4 shift is a little off but the7-8 shift is like the transmission goes into neutral, flares about 1k rpm and then slams into 8th. in manual mode if i get the speed up to 65 and shift to 8th then there is only a small flare and harsh engagement is this a mechanical problem with 8th gear or just a valve body problem controlling 8th gear. tramsmission was last serviced at 60k