aaronjheng Thank you for the clarification. Please note that IINA does not display a checkerboard background for all videos that contain an alpha channel.
Many DCC tools, including After Effects, treat ProRes 4444 as using premultiplied alpha, where the RGB values in transparent areas are multiplied by black. Compositing software understands this assumption and therefore displays the result correctly.
Video players, on the other hand, generally do not take premultiplied alpha into account and simply display the RGB values as they are. As a result, when the RGB values in transparent areas are black, the video appears to have a black background if the alpha channel is ignored.
For this reason, I recommend checking the result after compositing the video in a compositing application. You should be able to confirm that the alpha information is being exported correctly.
