Ask the seller to drive the kart with your go-pro to see if the issues were you or the kart.
Yes, you driving at partial throttle could be the cause. A list of possible causes would be: fuel pump, external fuel filter, fuel filter in the carb, carb floats, carb jetting, RAVE valve stuck open, low fuel level, poor fuel pick up, exhaust packing clogged, weak battery, fouled spark plug. That list is in no particular order of importance or likelihood.
There used to be a good bit of literature on the web on Rotax FR125 engines. Read as much as you can.
LAD Specialties customer / tony kart / rotax / kt100