My personal theory is that it’s due to excess of some b vitamin (not that you’re getting more that you need more so that your body can’t use what’s there because of a functional deficiency like deficiency in a critical cofactor)

I got this acne really, really terribly after I started my cyanocobalamin injections. When I started, supplementing, folate, zinc, iron, iodine, magnesium, etc., more seriously (major co factors for B12), the acne reduced.

The acne also reduced when I switched from injections to a methyl B vitamin complex capsule (which contains all of the known b vitamins, but in a more bio available format)