The banner system on OpenSea is extremely badly managed, I don't know why it is done this way but it systematically cuts your banner, like this:
To make all your banner visible, you just have to create a 2800x800 image, make your banner in 2800x264 format and place it in the center of the first image, like this :
There you go, you upload this image as a banner on OpenSea and you will have a clean banner. That's all.
I take this opportunity to give you the link to my OpenSea profile so you can admire my collection.