Artifacts with a Grid filter when video mode and original video aspect ratios don't match up

This is something I want to do:

  • Open a 1920x1080 60fps video in the Source tab or Timeline, set the video mode to Square 1080p 60fps, apply a Grid filter, change its parameters, and export.
  • Leave the top and bottom background #000000 rectangles in tact.

But every time I do this, some strange things occupy the black background rectangles.

Similar thing happens when I open a 16:9 video file in a non-standard custom 21:9 video mode, or a 4:3 video file in a 16:9 video mode.

Tested on v21.10.31 on Windows.

This is fixed for the next release.