Heard from a guildmate that when players join a group in the group finder & change their role once in the group, then leave. It will break the listing by making the original required role no longer be able to join even though the listing shows that the role is available.
Example: A group listing is looking for 1 tank, 2 dps. A "tank" (fake tank, forgot to change role to dps because they were doing a random normal or something) joins the group, then switches their role to dps. The listing will show that it still needs 1 tank, 1 dps, but other players who are actually tanks & set to tanks wouldn't be able to join the group & give them an error that says something like "Owner is busy" or something like that.