Connecting a Emulated GPS with an Arduflyer 2.5

Dear Community,
I am new on this Ardupilot module and in general I do not have any experience about UAVs.

I am emulating a GPS module, which is connected to a flight controller. In particular, the GPS emulator sends through a serial port interface (UART) working at 9600 b/s NMEA messages to the Arduflyer 2.5. Typically, the emulator sends around 5 NMEA messages every 1 second (i.e. frequency ~ 5Hz). The NMEA message I have used so far is the GGA, whose structure is: $–GGA,,llll.ll,a,yyyyy.yy,a,x,xx,x.x, x.x,M,x.x,M,*x.

As an example, I have successfully tested the following NMEA message: $GPGGA,225446,4503.937391,N,00739.545188,E,7,32,0.96, 0.00,M,54,M,*7A.

I am just interested in providing Latitude, Longitude, GPS status and accuracy. The other fields of the message are set as default, for example the height is 0 because I want to use only the barometer, and satellites number equal to the 32, i.e. the maximum value.

By means of the Mission Planner I observed that I am able to obtain perfectly a GPS FIX when I use a firmware version of the ArduCopter from v2.8.1 to v3.0. However, I am not able to obtain a GPS FIX when latest versions of the ArduCopter firmware are installed, i.e. from v3.1 to 3.2.1.

Anyone knows how to fix this problem? Does it depend on the NMEA messages?