Since this thread is actually active I thought I'd chime in that my 2019 Legacy has the exact same issue. I've tried tons of different cables; most of the time I get the "Smartphone connection error" screen when I first plug in, and then if I unplugged and replug it works about 75% of the time. It will also randomly disconnect when driving which is fun. This is with the latest firmware available OTA. Hopefully someone finds a solution eventually although I feel at this point it must be the firmware in the head unit.