My 2018 S with MCU1 and AP3 remains on 2020.48.37.6, while I see others on TeslaFi with the same configuration (and likely MCU2) on 2021.32.10.
It wouldn't surprise if the real issue is that the amount of available storage space on the eMMC near end of life simply isn't enough to hold the differential patch. I certainly can't even get the 7 MB owner's manual to open without going through all sorts of resource reduction hoops/restarts.