It might depend on which bus the instruments are tied to. In many aircraft, switching on just the battery turns on half the panel, and the other half comes to life with the avionics switch.
Edit: I’m on the stable build 1.29.30.0, and I get the same results. It appears that all avionics are tied to the main bus. With only the battery switch on, everything works, including the radios.
Do you have the same issue if you follow the OP’s steps to reproduce it?
YES
Provide extra information to complete the original description of the issue:
Avionics button is switched OFF but avionics are ON, when switching ON the battery flaps go to LDG position, when reducing throttle the speed is decreasing very slowly eg in real life when you put 17 manifold pressure with MAX RPM the DV20 is descending with 500 fpm and 65 knots speed but in the sim you must reduce RPM otherwise if you keep it like this and you do a pattern you have to decrease it to 15 manifold from the start of the downwind and if you are lucky in the base it will start descending with the desired speed, The trim is very steep, if you move it just one dot the configuration of the aircraft changes dramaticaly. The most important bug for me is the usage of throttle and RPM they have nothing to do with reality not even close…
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Do you have the same issue if you follow the OP’s steps to reproduce it?
Yes
Provide extra information to complete the original description of the issue:
Like OP, once I turn on the battery, the GPS, clock and transponder power up though the avionics switch is off. No beta version and no mods for this model installed.
If relevant, provide additional screenshots/video:
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