When in ac mode… What would make the cool air blow out the return/filter only, and not the registers? When in heat mode it worked correct?
Are you sure? Sounds near impossible but there is a chance that it has a variable speed airhandler with some type of fault with the indoor blower control circuitry (on the back of the motor housing) causing it to run backwards in cool mode.
You can’t run a single phase motor backwards unless you change the rotation connections (if the motor has that).
I can think of one remote possibility which would be if the run capacitor has failed and something (like a back draft) cause the fan to start moving backwards before it was energized.
Even if the motor is running backwards, it will not likely suck due to the inclined pitch of the blower wheel. .
I’m getting really out there now; but if you have a zone system and the supply actuators have failed to open you might get some stratified air flow both in and out of the return via the barometric bypass.
Was bath or kitchen fan causing a negative presure in the home ???
Was the supply air plenum insulated to the inside and the insulation has fallen off blocking the discharge on the fan creating a back flow.
How to tell if insulated to the inside just tap on the plenum and if no insulation it will sould hollow. There may also be marks from a spot welder for spikes that hold the insulation to the inside metal