If I’m correct in understanding that your horse will shift from a normal stance to a tightrope stance without external direction to move (ex on crossties) you may be seeing an attempt to relieve pain. In my (limited) experience, neurological horses “forget” to adjust their body when they move to a new position. Like leaving hind feet pointed sideways after doing a 180 turn. The proprioceptive signals are not getting through to prompt the brain “oh yeah, that’s uncomfortable, let me straighten that leg.”
Horses that shuffle into a weird stance are usually trying to alter the pressure on feet, or use different muscles, etc. because they do feel uncomfortable in a normal stance.
Did I understand your question correctly? I see I didn’t - you were asking about @fivestrideline’s grazing horse. I’m thinking that could be either or, and probably having a video of the standing normally and grazing in a circle around the non moving leg could be helpful regardless of pain vs neuro cause. I’d suggest keeping the whole horse in the frame as there might be some additional oddity as yet unrecognized.