The navigraph data should be the “Certified” range of these facilities, not the actual range that the instruments in the plane can give an INDICATION of navigational data.
MSFS does a half way decent job in calculating the signal strength from Ground based station, based on their TX Power, range, plane’s elevation etc, and this seems to be used to control any FLAGS – but the trouble is, when the signal is below Flag Threshold, there is still “unreliable” data being received by the plane, but MSFS acts as if that signal is non existent, when it is more than sufficient to be detectable and operate the Nav instruments in a limited way, and hence without a flag.
Any audio on that nav signal should also reflect that signal strength, and become NOISY with increasing range, but still be audible in the noise, when the flag fails to register an acceptable “Certifiable” signal strength.
Then there is the whole matter of what data is nav radio derived, and what data is GPS database generated, and how that is displayed to the pilot. 