The Navnet3D system has a failover action with GPS that I’ve observed, which may be relevant to your problem.
Typically there is more than one GPS data source available in a system. In my case it was the FA50 AIS and the GP150. The source to be used is selected as you step through the system installation process. In my case it was selected to be the GP150.
If during normal operation, the source of GPS fails then the system automatically changes to the other GPS source. In my case, the GP150 failed and the system started to use the FA50 AIS as the source of GPS. I knew nothing about the change, as the system just did it automatically.
It took some time before I noticed the GP150 had actually failed and was not supplying GPS data anymore.
This does not help with the question of why your GP150 is faulty, but may help in your understanding of what has happened in your system.
Furuno would not repair my GP150 so I bought a a couple of inexpensive GPS antennas with NMEA outputs and located them behind the instrument table. They are now my (dual redundant) primary source of GPS for the system.