What is the precise name/number of the device you use? Online I can find descriptions, that Tom Tom GO navigation devices connect to the Wireless Access Point that phones can start. This does not match what you describe (tethering via bluetooth).
One explanation could be that your Tom Tom can only receive data/internet when connecting to wifi shared by your phone. However, your phone is already connected to your home wifi and cannot start a wifi access point.
In any case, please give me a detailed name/version number of the Tom Tom GO and I will put it on the list of devices that do not work properly with FP1. Please keep in mind, that I cannot promise that we will actually buy such a device and test the connection.