I have a picture-in-picture video I need to clean up a little, because the fine details flicker when I move it across the background video.
If I use the filter Reduce Noise: Smart Blur on the PIP the background video vanishes. The filter Blur:Gaussian Blur behaves as I think it should: The background video is visible and not affected.
Confirmed. I reproduced this bug.
This also affects the new Remove Noise: Wavelet filter. I fixed both of these for the next version 20.06.