There are only 2 possible explanations:
1. This glitch is exclusive to parties. I don't know how or why but Naughty Dog needs to fix it ASAP.
2. The matchmaking server is not recognizing that some players have DLC when they do. This allows them to play DLC in custom games but are put in the non-DLC player pool when matchmaking starts. It only takes for the server to not recognize one person's DLC to put the entire party in the default player pool. I have already sent several of my friends messages asking them to re-download their DLC maps as a test to see if this is the case.
I was not finding DLC maps when I was playing solo or when I was playing in a party.
Yes, I noticed this last night while I was trying to find a Crushing Co-Op Arena match.
First game, someone picked Easy. Thought it was a fluke, backed out of the match and tried again.
I checked to make sure I had DLC toggled to "On" and then became quite perplexed.
Typically I'll get a Crushing game first time around, but I kept getting paired with new players.
Luckily I was saved when I got an invite from a friend and got a good game going
Didn't you read the patch notes, this is a feature in the this lab to see how removing DLC can **bleep** off ND customers, and to test our loyalty. The lab isn't only for gamemodes but for ND to play sik gamez.