Are you sure that you're connected to the correct ethernet port, the one directly next to the power socket (the other is WAN)?
I've flashed by MAP-AC2200 that way and it's been working fine for me (I'd recommend master- or 21.02 snapshots due to the fixes for its power levels). It's correct that if anything has gone wrong, the only way to recover this device is via serial console (it has a recovery mode, but it doesn't work/ can't re-create the UBI partitioning it expects - and which isn't shared by OpenWrt).