Turn off the water to the tank, as see how low the water goes.
If it stops at the flapper, that it
If it keeps dropping below that, it could be the seal between the tank and flush valve.
Or if you have the fill tube from the fill valve too far down the overflow tube, it will siphon water from the tank.
Fluidmaster and Korky have little metal parts that hold the fill tube above the overflow tube to prevent siphoning.