It really doesn't have to be anything 'magic', remember that pressure decreases at higher altitudes, and increases with an ejection charge. So the two events are in opposite directions. When my A2/A3 is in a part of the rocket that will see an ejection charge the charge shows on the graph, as a sharp negative altitude spike. But apogee will be the part where the graph levels-off between ascent and descent. So if the charge occurred exactly at apogee you might get a dip where the peak of the curve would have been, but since the rocket's altitude change is relatively small at that point it shouldn't impact the apogee reading by more than a few feet I'd think.
But yes, it should be taking pressure readings from the moment you hit Record in the app until it considers itself landed. I think it might use its accelerometer for launch detect, though I'm not certain, I've had a few flights recorded as ~100ft and only a few seconds long because it considered launch and landing to have occurred before the rocket ever left the rail, not sure if it was because of a wind gust or some other shock to the rail (or maybe loading the ignitor).