Inclusive roaming in the US allows for calls & texts within the same country and back to the UK to be zero-rated.
Were these texts sent to standard UK or US mobiles, and not any 5-digit shortcodes? Have you been similarly charged for calls? Were there multiple texts to the same number, and if so - were some zero-rated but not others?
If person-to-person texts to UK/US mobile numbers were charged with an inclusive add-on present, then that sounds like a billing error.
