Maybe the KS is cut too short?
throw a toe strap around the wheel and down tube that will help too...
I fit a webbing piece and a side release buckle for my Porteur rack fitted bike.

SOME OTHER SOLUTIONS

Have a Koga trekking Bike , they fit a 2nd KS onto the lower end of the left tubus lowrider . that works OK.

the HD frame I got built for touring has a Nylon Block under the down tube,
It has a function as a stop for the fork crown to bump into, and the Front derailleur cable housing stop.

German Tout Terrain frames have a stop on the headtube, and a clamped on stop on the steerer tube that hits that stop.
Peter White the US retailing importer of those ..