Something doesn't seem to fit here.
How would tree debris affect the evaporator drain?
Air circulated by the heater/AC goes through the cabin air filter. That's fine enough to eliminate any significant material that could block the drain, especially in such a short time. (In part, that's what the filter is for, which raises the possibility that the filter might not be installed, or intact. Did the dealer check it?)
The drain exits outside the car, somewhat behind the right front wheel and in toward the center of the car. It, itself, isn't exposed to trees except to material that is on the ground and might be kicked up as the car is driven. But this wouldn't have anything to do with where it's parked. All cars drive on streets lined with trees. This is not likely to cause repeated problems. (Incidentally, when the AC is on, there is usually some dripping out the hose which is easily seen. This is one way to roughly check if the hose is draining the evaporator pan.)
Some blockages that have been reported were due to insects crawling up the drain hose and nesting. That could cause a blockage. But, again, it's not likely to be repetitive, unless there's an overpopulation in the area and they have a penchant for Subaru evaporator drain housing. (I'm not intending to make light of this; rather, my point is the unlikeliness of repeated blockage in such a short time due to this particular cause.)
It should be noted that the water runs down the evaporator drain hose. As such, the interior and bottom of the hose is wet when the AC is operating. If you constantly drive on very dusty roads, it's possible for dust to collect near the bottom of the hose (it sticks to the moist surface) and build up over time. This could be a cause of repeated blockage. But it depends on your driving, and we don't know anything about it.
Also, do you normally operate the AC in "recirculate" mode? This is helpful for rapid cooling down of the cabin, but I would recommend not using it normally. When outside air is being drawn in (instead of recirculating the cabin air), air flow in the heater/AC duct comes out the vents in the cabin (as usual), but a small amount is also pushed out through the evaporator drain hose. This flow of air down the hose can help to keep it clear.
In summary, it's not normal for the evaporator drain hose to become blocked; and while there have been reports of blocked drains, there's usually some other factor involved. Finding that could be the solution.