Hey OP - thank you for the post. We investigated this and want to share what happened here.
We found a bug in our patch system where a small percentage of players were not able to properly obtain the latest patch, meaning this percentage of players were not getting matchmade with the larger playerbase who are on the latest patch. This lobby formed as a result of that bug and since there were so few players playing on the older patch, you all ended up playing together.
We're actively rolling out a fix for this on the backend and have a fix for our patching system that will go live with our next title update so we can prevent this from happening in the future. Hope this provides some insight and apologies for the bug!