In the cessna 172 when you turn on the master battery switch it automatically starts the primary g1000 display. This shouldnt happen as the primary g1000 should be started via the avionics 1 switch. The actual avionics 1 switch turns on/off nothing.
I don’t think what you are describing is the correct logic. Avionics/Bus1 is not dedicated to the PFD while Avionic/Bus2 yes turns on (among others things) the MFD.
You can search for videos on YT that show how to start a C172 G1000. Also I am pretty sure some users with IRL knowledge will provide additional feedback (or will correct me if I am wrong 
).
Real-world we turn on the PFD prior to engine start using the standby battery switch, before the mains (and downstream avionics busses) are even switched on. This powers the essential bus, which energizes the PFD. The essential bus is also energized by the main battery bus, so the PFD will turn on when the master switch is activated.
However, the MFD will stay off, along with NAV and COMM2 capabilities until the avionics 2 bus is energized, and the avionics cooling fans won’t come on until bus 1 (front) and bus 2 (rear) are activated.
The only real thing the avionics 1 bus does is activate the front cooling fan and any additional, optional avionics equipment installed on that bus (which is not a thing in MSFS). I guess it could be argued that it could allow some load shedding from the essential bus via the circuit breakers, but I can’t see a use case for that because if you’re using the avionics bus, the ESS bus is also powered. Maybe useful to isolate if the STBY bat is shorting past its relay and overvolting the ESS bus? But that’d be like a 1:1,000,000,000 thing and the stby battery feeder has a breaker, anyway.
Let’s just make it easy and say the Avionics 1 bus powers the front cooling fan.
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So no issue with the actual logic right ?
I don’t think so. I’d have to test more fully while pulling circuit breakers and flipping switches. But as the topic author describes it, it is not flawed for the PFD to activate with the main battery switch (unless the main battery is dead).
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Moved out from Sim Update 4 Beta | MSFS 2024 since not beta related.