Are you sure this is the button you are pressing? It should not be even possible until you are already at the final approach fix anyway.
“Activate Approach” in the flight plan is NOT the same as doing this, and happens earlier in the process. There is no such thing as a glide slope in an RNAV approach. The glide path is a simulated glide slope that uses something other than a ground radio beam (something like GPS or baro altimeter readings) to make your plane descend at a 3 degree angle to the runway. this is indicated when you see “GP” in the flight mode annunciator.