Actually not that unusual!
Our Grob 109B (powered by a Grob 2500cc 95 hp Grob 2500 engine) gives a 100 rpm INCREASE when carb heat is applied at most power settings, and in all temperatures and weathers, whilst on the ground.
Close inspection of the POH reveals that the Before Take Off Checks require selection of full power (to check rpm) and THEN to apply Carb Heat. Low and behold there is a 100 rpm LOSS.
Selection of Carb Heat in the air gives the normal response.
UV