Quick interaction query - new player

Hi, I'm a new player who last night went to their first local tournament using Brandt Gate, namely a prison deck. In my second game we came to a weird interaction that I wanted to know the ruling for since nobody there really knew what would happen.

When using an effect like ABP Seraph Snow's effect (C Blast 1 to put two Rear Guard in prison), what happens to rear guard that have been OverDressed? Do all cards in the slot go to the prison, does only the OverDressed card go there and other cards remain, or does the OverDressed card go to prison, and the remaining cards go to drop?

I know this is very late, But when an overdressed unit is moved to a different zone, All cards underneath it go to that same zone as well. Im sure you know by now but I hope this helps anyways :)