I checked the version of ReGBA running on my GCW Zero as well, and this has the same graphical issue as on the PocketGo.
It's ReGBA version 1.45.5, Nebuleon/ReGBA commit 6762f6b.
This version of ReGBA doesn't have a border but it allows you to increase the scale of the display to fill the screen.
The version I have running on the PocketGo is nearly the same, but lacks a commit number and has a forced border display.
It's the one featured on the stock firmware with the SD card provided by retromimi.
That said there's other graphical issues I've noticed like the sun beam effect on the title screen, there are incorrect colors when the sun beam crosses over the sun graphic in the sky.
Retroarch recently got an early release for OpenDingux, and mGBA is one of the libretro cores that is confirmed working, but I do not know if it's fast or slow as I've not yet tried it. I'm guessing it may be slow due to processing overhead from added accuracy, but that may fix the graphical issues.