Analog Stick Issues - Help Please!

Hi all, So I’m pretty new to RetroArch and emulation in general, hoping all you cool people can help me out.
I’ve been running 1.4.1 on Windows 10 to play SNES games for a few weeks with no issues, but when I went to load up a couple N64 games on Mupen64plus I ran into a snag:

The left analog stick works correctly when pushing up and right, but left and down will only produce “walk” commands. For example, in both Ocarina of Time and Super Mario 64, pushing up and right on the stick will allow me to run or walk depending on how far the stick is pushed, but left and down will only produce walking.

I’m using an original Gamecube controller plugged into a Mayflash GameCube Controller Adapter into my PC. Tried using 2 different controllers with no luck. I’ve tried re-calibrating via Control Panel > Devices > Mayflash Adapter Properties, and it shows full range on both the X and Y axis.

Any advice you could send my way is very much appreciated, thanks!