I just had mine apart to do some water proofing. This is a 2020 SRS premium. The chargers themselves are closed up boxes, with two amphenol connectors for in/out that look really water resistant. High quality stuff. If water goes through the cooling fins sideways and touches the fans maybe that could be the cause. The chargers themselves looked tip top to me.
Mine went two years with no trouble in rain, then became very easy to get an Isolation fault in even a light rain. My guess is the rainwater goes in at the top of the battery case where the HV cables go down in from the chargers. I guess time will tell if that was a good guess or not.
Here are two pictures. First is the gap before I filled it, shown with the charger out of the way, and the second is the same with a bunch of silicon sealer on it. In retrospect I wish I had done the interior of those wire wraps on the cables too to reduce water wicking but I didnt think of it until it was reassembled.