The video thumbnail is set to the image at the beginning of the video (at 0:00).
Therefore, if the beginning of a registered video is completely black, the thumbnail will also appear completely black.
※ We have confirmed a phenomenon on some Android devices where the screen is completely black immediately after recording starts.
The thumbnail can be changed to any image of your choice, so please replace it as needed.
Related: Edit video thumbnail (Web Browser)
Notes
Even if you cut the black portion at the beginning of the video using "Trim Section," the thumbnail will not change automatically.
If you wish to fix both the video and the thumbnail, please perform both "Trim Section" and "Replace Thumbnail."
Related: Remove parts of video (Web Browser)