1. The libretro core is still on 0.7.0 and not 0.7.1
2. It was fixed for the Switch standalone frontend, the input handling for libretro is separate(it affected both platforms, because the Switch gyro code was copied directly from the standalone build), so it won't be fixed by updating the mgba core.
It's a super easy fix, just the addition of a minus. I wanted to do pull request after endrift confirmed my suspiciouns on what's wrong by fixing it, but I was always to lazy until you reminded me:
https://github.com/libretro/mgba/pull/137