I was suggesting to remove all t stat wires. If the problem still continues u know the backfeed is in the furnace board ie a jumper. If not the backfeed is in the t stat end. And if you jumper between r and g the furnace fan will come on. If you jumper from r to y then the Ac and furnace fan will come on. The exception is if you have an old furnace with no y. Then you would have to jumper between y and g on the t stat to force the Furnace fan on when T stat was calling for cool. This causes a problem because when the t stat was set to summer fan on, or hrv went to on the Ac would come on. As I said op says no jumper is installed. In that case a relay needs to be installed in order to remedy the problem. If I'm incorrect what would the y terminal be for. The y terminal allows the furnace fan to kick in when cooling is called but isolates the condenser contactor when the fan switch or hrv connects r and g. However op said this furnace has a y position on the board.
Op sorry to hijack your thread hope we are able to help, I know how frustrating things can get.
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