The PNG frames are transparent. The exported video has a black rectangle. Running background removal again seems like the obvious fix, but it can be the wrong step: the alpha channel may have disappeared during encoding, or the player may simply be ignoring it.
Source: [Dev.to](https://dev.to/river_jiang_670cccce7d97d/convert-a-png-sequence-to-transparent-webm-with-ffmpeg-3ggd)