Seeing as this appears to be the only existing evidence of anyone online having this issue, I'm bumping as I have the same problem. I would love to have one of these work in my car, but the one I purchased (MMB Max) clearly is not compatible with the OEM headunit in a 2020, the seller has said that there is a firmware update yet to come which will add the OEM headunit to the list of headunits compatible, however the manufacturer (JoyeAuto) has not gotten back to me confirming this.
I'm curious if OP had any luck finding an AI Box which does work with the Subaru head unit or if they discovered anything else regarding how to address getting one of these to work with the stock head unit, it is rather frustrating to me that I can't seem to find any reliable info about which specific vehicles these things are compatible with but, that's life shopping for these sorts of things I guess
Thanks!