Unexpected rectifier waveform
This has happened to me once and I struggled with it only to find that what I thought was a 1N4148 glass diode (like a 1N914) was in fact a 3.3V zener diode - try reverse biasing it like a zener to see if it draws current and acts like a voltage regulator.

Even if you assumed 10pF cross capacitance this would be an impedance of nearly 16 Mohm at 1kHz and not enough to do what the screen shots suggest. It has to be a broken diode or the wrong diode.