Resolve 15: Invisible Power Window (video inside)

It's a very simple project.
One time line
One video file.
After having added some power windows for mosaic blur usage, I noticed a mosaic in a part where there were no power windows.
I tried ALT + H, but no success.
I even deleted all power windows, but the mosaic is still there....
It might be something very obvious, but I'm missing it.
Video below:

Can anyone help?
One time line
One video file.
After having added some power windows for mosaic blur usage, I noticed a mosaic in a part where there were no power windows.
I tried ALT + H, but no success.
I even deleted all power windows, but the mosaic is still there....
It might be something very obvious, but I'm missing it.
Video below:

Can anyone help?