There's two ways to do multi party messaging - Broadcast SMS, or Group MMS. The later is more likely here, given it's a regular group message. It's most likely

There's two ways to do multi party messaging - Broadcast SMS, or Group MMS. The later is more likely here, given it's a regular group message. It's most likely to be a bug on one of the MMSCs, but could be a problem elsewhere if the message is getting corrupted in flight. Alternatively, it could be routing issue in a certain set of numbers.