I get one on my right cheek sometimes, both on my rollers and on the road. Finally figured it out: my right leg is maybe 1cm shorter than my left. I shimmed the right 3mm which helped a lot. I also moved the saddle nose to the left when the sore has been active, just a few degrees.

This saddle sore thing only started in my mid-60's. Never had a problem before. Not the saddle or position, other than the shorter leg, just slower skin healing with age. That said, they've healed a lot quicker, enough that I can ride every day, after I started putting Tolnaftate cream on my problem saddle area every morning. So maybe there's a fungal component in my case.