I don't know if you want to call it a bug or bad design, but when you arbitrarily and capriciously drop my merc and won't let me repop it because too many other people enter the zone? That's just unacceptable. You are deciding my solo gameplay based upon the conduct of others? What happened to instanced zones? Or limit it to a number of players to allow mercs. Absolutely inexcusable. Yeah, I said it.
It's an old and known intentional mechanic. Most don't experience except when raids are gathering in popular zones.
This^^^^^ Happened more frequently when there were more populated zones. I used to run for the zone line when I saw that message lol
This mechanic exists because zone performance starts to break down when there are too many players in the zone. It is meant to encourage you to move to a new /pick. I agree it is disruptive to have your mercenary removed.