Inaccurate neutral stick position, and down-left issue with GCN controller

[SnifferDogBen]

There is an issue with this plugin in RMG (I am using RMG v0.7.0) where all the values are skewed towards down-left slightly. The effects of this became more noticable in gameplay when increasing the stick range by editing the mupen64plus-input-gca.toml file.

I have tried multiple controllers, all of which have this issue. Removing the deadzone (setting it to 0) results in mario walking down-left slightly.

In other N64 emulators such as Project 64 using plugins such as Wermi's GC Input Plugin (pj64-wiiu-gcn), the controller's neutral position is completely fine and is true to the values of the controller itself. My controller is also fine when using Dolphin. For these reasons, I strongly suspect this is a software issue.

Please see screenshots below showing results of a controller test using mupen64plus-input-gca input plugin. I have compared against ideal N64 Hori controller angles because it showcases the down-left issue better.

Everything else about this plugin seems perfect, so its a shame that I am unable to use it due to it affecting gameplay.

[SnifferDogBen]

I should also mention that I have tested the plugin with multiple Gamecube controllers (that I know to be working and dont have any deadzone or drift issues) and the same down-left issue is present.