I downloaded v1.04
It contains a "fat plugin" binary with both x86_64 and arm64 segments present.
There is definitely a glitch with the signed status of the file.
It claims it is unsigned yet attempting to sign it with my own Apple Dev cert fails.
I was able to get the plugin working by extracting the arm64 portion of the plugin and renaming some files.
cd ~/X-Plane 12/Resources/plugins/XMidiCtrl/mac_x64
mv XMidiCtrl.xpl fat.xpl
lipo fat.xpl -extract arm64 -output XMidiCtrl.xpl
After extraction X-Plane 12 starts without error and XMidiCtrl was present.
The lipo tool might not be available if you don't have Xcode installed...