I have been in groups where we continually requeued random dungeon even though only 1 dungeon is available to our level bracket. When I tried to make a group that could only que for 1 dungeon it would not allow us to que for a random dungeon.
Is this because it only works for dungeon finder groups or have they changed something to prevent speed leveling with prison spam and other fast dungeons?
Seems like you can't have one that is the lowest level to enter the dungeon to do the trick for some reason. Not sure this is actually how it works though.
Prison spam worked in a way that you had to have 1 person near max level of the instance and 1 person near minimum level of the instance. Since there's no other dungeon in that level range queing up together for RDF would always pop BRD prison.
And I figured, but don't quote me on this as I'm not sure, that IF player levels are set up that way you would only need to kill the right boss (the chick instantly to the right as you enter) and get RDF completion and the loot bag. That's likely because other NPCs in the dungeon are a lot higher level than the low level player.
If players' levels were closer to each other (as opposed to minimum and maximum levels for BRD prison) you'd be forced to run the whole dungeon that could take easily up to an hour to complete.
Now, I'm not sure if it's fixed or not but I'm leveling a character now so once I hit the level requirement I'll post feedback if I remember.
BRD prisons level range is 47-57 and I can confirm you need to be 48 to que random when you have a 56+ in your group so you can only get into that dungeon.
This is just a theory but I believe it has something to do with the mechanic that prevents you from queing random when entering a new expansion. In Outland the first dungeon is 58 and in Northrend it is 68. Wikis say you can't que for random at 58 and 68 since you only have 1 dungeon available but since the next dungeons are available are always 1 level above it could just be coded to not allow you to que random if you are at the lowest available level.
TLDR: You can't que for random when there is only 1 dungeon available and someone in the group is at the lowest possible level.