My bolts still look good on the ST40 calipers. I regularly spray somewhat harsh cleaner (50% diluted grime reaper) on it pretty regularly.
Yeah, maybe you got bad bolts. Or they were dinged up during the install. I would remove the bolts and let them soak in some rust remover. Then maybe use some zinc spray on bolt heads.
2013 F30 328i Melbourne Red
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red