Fredkh

This sounds like a genuine dungeon progression bug, especially if the Mistress of Mane normally spawns immediately after all the Gardevoir are defeated and it has happened on multiple runs. I’d first double-check that there isn’t one Gardevoir stuck somewhere or slightly out of the normal area, then wait a few seconds after the final one is defeated. If she still doesn't appear, I’d leave the dungeon and try another run to see if it happens consistently. If it happens again, definitely take a screenshot showing the cleared room, and ideally a short video if possible. Also note the date/time and whether you were playing solo or with other players. Those details could make it much easier for whoever maintains the dungeon to reproduce the problem. If other players are having the same issue, that would also point more strongly toward a server-side or scripting problem rather than something specific to your character.