Don't see my thumbnail with an mp4 video

I like to add an thumbnail / poster to my mp4 video, but i only see the actual video.

This is the code in the page:

And these are the files located in the page directory:
test.mp4
test.mp4.thumb.jpg

What am i doing wrong?

Thanks in advance...

Can you please create an issue on the Grav repo and we can investigate this: https://github.com/getgrav/grav/issues