It is not a bug. 'A card with Overload' is supposed to be a card with a printed, 'naked' Overload. This card 'gives' itself Overload in a way. This is basically the same reason why N'Zoth doesn't resummon an Unearthed Raptor. I know, this kinda makes no sense since intuitively, this is an Overload card.
And for someone who asked: Tunnel Trogg says 'whenever you Overload' so it counts Overloading mana regardless of how you Overload.