This is a key point also. 50X16 will be spinning your flywheel very fast with a lot of inertia. When the power drops off it will take a while for that to settle in, combine that with your legs letting off the gas and I think you’re seeing some extreme yo-yoing of power in your trainer as it tries to find equilibrium. At @RONDAL suggests, try a test using low gearing and trying to keep cadence steady and see if that improves things.