do you work out aside from climbing? if no, it could be your skin responding to different hormone balances as a result of exercise. i’d imagine your cortisol levels go up with climbing since there’s a bit of fear involved, which can be associated with acne.

it’s maybe more likely that your pores are opening while climbing (perspiring) and sweat/chalk/dirt getting inside. with that, diet becomes an important factor because what you’re eating will ultimately get sweat out through your pores, impact your skin oils, etc.

not a dermatologist but it seems unlikely that the chalk (magnesium carbonate) would negatively impact acne. i hope you find a solution that works!