This issue has to do with the selected aircraft’s aircraft.cfg. You should look inside if it supports NAV1 standby. This is the Radios section from the default 172:

[Radios]

// Radio Type = availiable, standby frequency, has glide slope

Audio.1 = 1
Com.1 = 1, 1
Com.2 = 1, 1
Nav.1 = 1, 1, 1
Nav.2 = 1, 1, 0
Adf.1 = 1
Transponder.1 = 1
Marker.1 = 1

As you can see, the first two values should be 1, if the second is 0, then there is no standby frequency, so no swapping. We suggest that you retest this with a default aircraft, like the 172, but not with a modified aircraft.cfg.