Thumbnails of portrait videos are shown with a vertical black line to the right, like if it were part of a black photo frame, check the following:
Now when videos are Not Portrait, thumbnails doesn’t show this weird black line.
I’m not sure this is happening only on portrait videos, I have not checked all the videos in the account, anyway Why is this happening and how can I avoid this?
Thanks in advance.
May I ask where did you embedded your video? Or that’s the original one?
Could be due to the aspect ratio or something 
Kindly, see if below post helps you to resolve your issue:
Hi, thanks for your answer.
This thumbnail is the original from:
https://customer-XXXXXX.cloudflarestream.com/ZZZZZZZZZZ/thumbnails/thumbnail.jpg
So far each video thumbnail I have checked has this vertical right black line, if the video aspect ratio is portrait… I don’t understand why, because the video itself doesn’t have this line.
I have had already checked that post you reference before, but no luck, as my problem is not related to embedding.
Hi, there.
I am having the exact same problem and it is a bit annoying.
It also only happens to vertical videos somehow.
The thumbnail will have that black vertical bar on the right side.
It happens to normal thumnails and also animated thumbnails.
Hi, I already contacted support about this and the fix will be released in a future product update, although there is no timeline provided.
Anyway as a temporary solution you can customize the size of your thumbnail and this will remove the black line, just setting one of the sizes is enough to get rid of the line, only height=360 or width=240 is enough.
https://customer-eusmn1jvxxtxsst3.cloudflarestream.com/a23cc3916db26b003cd2751d06453b44/thumbnails/thumbnail.jpg?width=240
https://customer-eusmn1jvxxtxsst3.cloudflarestream.com/a23cc3916db26b003cd2751d06453b44/thumbnails/thumbnail.jpg?height=360
Thank you very much for the quick answer 
!
Got it, will use your temporary fix in the meantime 