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The only ways this symptom can happen is..
short in wiring transferring power to other wires
back feed through a common wiring point.
The actual best way to begin eliminating is to replace the thermostat wire from the furnace to the thermostat, furnace to the ac.
I know it does not sound fun...but you have 2 choices...keep hunting for the culprit , or start making certain options go away.
Ok, if you have a Bryant the Hum terminal is only live while the gas valve is energized. So to test the humidifier requires the burners to be on.
I have had humidifiers backfeed depending on how it is wired. The issue is if nothing has been changed and just popped up. The humidifier should be wired to HUM terminals, AC to Y and C....It is when we start combining items on the common side that this feed back can happen.
Take everything off like ac and humidifier and just have R and W from stat hooked. Run that a couple cycles, then put a volt meter on HUM terminals for 24 v during heat cycle. If so hook up to HUM. Run a few cycles. Then instead of putting the yellow from thermostat and the red to ac on the Y terminal, just wire nut them together. So Y from thermostat and Y to ac wirenutted . Then the last ac wire to C.
Hopefully onto something there.....The key is to breathe slowly and not get frustrated...or you keep starting over....dont ask how I know that....
Ac is Y/Y2 and 24vcom