Rae, I’d say this is just how the Renpho bike works. Although maybe yours is worse than mine? This morning I was pedaling at 70 RPM at about 140W with a smooth pedal stroke. But if I push a higher wattage, say over 300W, I definitely notice a more jerky pedaling experience. I find a higher cadence results in less relative resistance (in ERG mode) which gives a smoother pedaling feel.
I’d say the Renpho bike is fine for beginner/intermediate riders, but people pushing high wattage on a regular basis might be better off with a higher quality (more expensive) spin bike or smart trainer.