Attilius Mansion (Tarantia Commons) and Slaughterhouse Cellar (Thunder River) dungeons are solo dungeon even in epic mode. If a group tries to enter in epic mode, all players will be splitted in different instances.
I doubt the epic version of those dungeons were intended for solo, even cause the large amount of trash mobs, so I think it was a mistake, a bug, something not working as intended.
Except allowing groups in epic mode, there would be nothing to adjust in the epic gameplay and it could be a low cost idea to refresh an old content.
P.s: …and even a way to find a place where to put the “handpainted column” recipe (architect recipe - pillar in aquilonian style), one of the few decoration recipes none was able to find in years (I wouldn’t be surprised if the recipe was in the game and really epic Attilius would drop it).