Two things that can fix this.

First. Create a default C172 flight with everything off except the Avionics Master switch.

Second. Create a keyboard combination for the Avionics Master switch. I use the combination CTRL+SHIFT+A for mine.

I have actually done both of these. But on some planes I want the Avionics Master switch left off.

Also. Pressing CTRL+E (default engine start) also turns on the Avionics Master switch which is why you didn't have the problem when you started it that way.

Hope this helps.

Mark