How often did this happen? This is the first time this has been reported.
Normally when the LL line has a lot of guests in it, CMs will load more LL guests to the attraction than standby guests.
We'll see if others share the same experience but I would wait to see what others report. Are you still there now? If so, I would find a guest entering the standby line at the same time you're entering the LL and then see when each of you boards the ride.
I find it challenging to believe the Disney will do this as it will stop people from buying LLMP (as it's done to you) and they would lose this revenue. I'm sure they don't want to do that.