We try not to engage the auger on our 1680 at full RPM unless it is empty. To do it full of grain smokes the belt, and then the slipping starts. I don't notice it on the 2188. You probably need a new belt, but adjustment might be worth a tryIJ
Sure wish we could do that in grass seed. I dont care much for starting at full pedal, but if we dont, it'll either shear a pin or fog the belt. Several folks have stripped the flighting off the vertical shaft trying to unload at less than full throttle.