A background-removal API can finish successfully while the exported file is still wrong for the next step. A video that plays on a black background might have lost its transparency—or the player might simply be displaying it against black. Those are different problems.
Source: [Dev.to](https://dev.to/bill_king_d4cd78085ee37d2/transparent-video-exports-why-one-mp4-url-isnt-enough-3kko)