Either a light leak (if it was supposed to be dark) or some obstruction/vignetting, together with an auto-stretch in the viewer. Quick guess there could be an OAG placed on the short side of the main camera chip. But auto-stretch makes a 10% vignetting look like a black-white transit....
I did use a ha filter with telelens during daytime . I made the same test that afternoon in the Windows environment using SGP. With no problems...... I will test again using the rpi of one of my astro friends.
The way Ekos stretches the image is quite extreme so you probably get a better comparison by taking an image via Ekos and another via SGP and open them both in PixInsight or another application that can stretch FITS files. That way you get a better comparison of the images and can see whether there indeed is a problem or not.