All of that is by design: OpenMPT does not do Amiga-like stereo separation by default. At 100% stereo separation, panning is applied 50% left/right like in other PC trackers. And stereo separation in the Mixer settings is not based on channel panning but on the entire audio signal, meaning that anything beyond 100% will expand the stereo stage through phase inversion.

As the stereo separation feature is working as designed, this is essentially a duplicate of 0000316.

This is Stereo Separation set to 200%: https://streamable.com/rjitqk
Listen to the right channel and please notice when the bass guitar enters there. In fact it plays all the time in the left channel. So there is more going on there.

Updating my example:

With 100% stereo separation set the channel 01 is leaking to the right, but no channel 04 is present.
With 200% stereo separation set the channel 04 is leaking to the right, but no channel 01 is present.
And vice versa.

Yes, there is a 800 command, which sets the panning fully to the left.

(Panning commands 8xx and E8x can be disabled via playback compatibility settings which would normally be determined heuristically, but whether to enable that settings or not cannot be detected 100% reliably for all modules)