Getting dropped from a group ride is nothing exceptional, it happens all the time, whether because someone has overestimated their own ability, the group has got carried away with itself, the information was wrong, you're having a bad day or someone just wasn't paying attention and missed a turn. Or even because someone shot off the front and got lost! What matters, if it's advertised as no drop, is what happens next. There are various ways a group ride can deal with the situation, most obviously regular regrouping and/or having a designated backmarker or "babysitter" who knows the route, is patient and encouraging, good with mechanicals, etc.