Photomerge Drops Edges when blending layers

Hello, I'm on PS 25.0.0 and I use a Macbook Pro 2019.

I scan my larger art with the Epson 850 Perfection Pro scanner. And I use PS Photomerge to assemble/digitize my art and it does well except for one thing: It leaves off slim slices of the edges so I have to crop my image smaller to compensate.  And for art, this is oftentimes a bad idea. 

I have another way I do it but it's tricky. I scan so that I leaves space around the corners but the images have to be aligned well (90 degrees or very near) for Photomerge to work.  When I've scanned and aligned them well Photomerge merely eats the edges that I left off to begin with in the scanning. 

But I wonder if I'm missing something and that  there could be a more realistic solution? I've been scanning things this way (leaving blank edges when scanning) for years. 

Keep in mind these are Not photos but art - usually watercolors on paper. 

Many Thanks for any Assistance,

Frank B.