If on stbemu on firestick go to settings and change aspect ratio

Same happens when I'm using the app on a Firestick, but not on my Droidbox. Same thing with the Helix app too on Firestick, but not with Perfect Player. Never figured out why, but also never needed to when I have a viable workaround.

Press menu button on the Firestick remote then select Aspect Ratio -- you can leave it on auto but that may not work correctly for all channels so you'll have to manually select 16:9 or 21:9 ratio.

I usually leave it auto which shows 4:3 or 16:9 correctly and then manually switch to 21:9 if I need to