Green bars with croppadd and canvas

I was pointed here from this question.

When using the croppadd video filter in order to be able to place subtitles underneath a video, the resulting bars at the top and bottom are green, when one would expect them to be black.

I am using Windows 10, Version 2004, and VLC Version 3.0.11. To reproduce, active the croppadd filter and choose to padd, say, 60 pixels from the top and bottom. Alternatively, use the canvas filter with the "pad video" checkbox enabled. When afterwards opening a video file in .mp4 or .mkv format with video codec hevc or hev1 (but not e.g. avc1), the resulting bars at the top and bottom are green and not black. Subtitles are displayed in the green area though, as desired.