If you invite a player that is already in a party you will be put in a "invisible" party and you will always enter private instances completely alone. But after somebody else joins you in that party you can then leave it and be set free from this bug and be able to enter dungeons with random people again

This happened many times before to me back at the easter event but I didnt had a way to recreate the bug , and being a warrior alone in that event was really boring and infuriating...

Although annoying I believe that it could have a great use for many people to be able to enter dungeons alone , for example when doing dailies and you are put with people that has speed sets and 0 reasons to wait for you