Rush miniature image

I've created a short clip with Rush but are very confused about the way it selects the Miniature or thumbnail image.

Saving to my local computer there are no way, as I know to select the thumbnail image, it is selected by Rush. I expected Rush to select one of the starting frames as thumbnail but no it select a frame in the middle of the clip .... ???

Tried exporting to YouTube here is an alternative during export to either select a special frame for the miniature/thumbnail image or select a file on your computer. But Rush do not go along with my selection instead it selects the first frame independent of my settings.

But then post any of those on Adobe Portal then Adobe Portal des regards the miniature/thumbnail immage and selects its own miniature/thumbnail image in the middle of the clip ... ???

What is going on?

Rush version 2.7.0 version 51 on Windows 11