with the newest LibreELEC (8.2.1 kernel: Linux 3.14.29)
FYI, v8.2.2 is the newest version, although that will make little difference with the problem at hand.
The video plays fine here too on Ubuntu + Nvidia graphics on both 21:9 and 16:9 screens.
which is an Eminent EM7680 4K TV Streamer
Eminent does have its own helpdesk, and Eminent also releases their own builds of LibreELEC. Hence they might be behind in version numbers.
So I suggest you contact them first, since you also paid them for the Eminent box.