HCG will raise LH and FSH by stimulating leydig cells, and I believe it can also raise estrogen and progesterone (to what degree, I am uncertain lol)... introduction of any sort of gonadotropins, synthetic or endogenous, can lead to acne. Any change in hormones can, really. It's not like endocrinology is a walk in the park.
If you're on TRT and your treatment is successful, I would focus on minimizing the acne and being happy with the results.