Currently the thumbnail appears to be the first frame of the video.
My videos fade to and from a brand colour, so all the thumbs are the same.
Feature: In layer settings, choose Thumbnail Position: fraction of duration (0 - 1) where 0 is first frame, 1 is last frame, 0.5 is centre frame. Clip settings can be "Layer Determined" or individual.
Positions other than 0 or 1 are approximate as some formats are not easily seekable to exact positions.
This should be changeable using the web api.
If changed by any means the thumb should be recalculated.
If the file format does not allow seeking and the duration is long then maybe there is a max offset frame of 1 second as a fallback?