Everything that produces light (radios, exterior lights, gps thing, etc) just peaces out after around 15 minutes of flight. This is not specific to the C170B, a few other airplanes do it.
You do not happen to have an Alpha Yoke, leaving the starter in the on position, do you?
This is a known bug discussed in the main thread. The alternator is supposed to come on with the battery switch, but it does not.
Workaround at the moment is to use the keyboard command to turn alternator on. It has to be the command ‘alternator on’ and not the ‘on/off’ toggle from what I have experienced.
This and the CHT bug should have easily been caught in testing, no idea how they slipped through.