Came here for this exact bug. I have this as well. I haven’t tried it on the ATR-42, but on the 72, the prop brake stays on no matter what. Just like OP, the prop brake switch does indicate off, but the light “PROP BRK” is illuminated. I tried turning off all the hydraulic pumps as @DefeatedThree7 mentioned but that did not work for me.
However, as others have mentioned, its just a visual bug since the prop brake doesn’t actually stop the performance of the engine (nor does it when it is working properly).