Just updated to the firmware version 378_4129. (@maylyn thanks for the info!)
1. Same bug (found on 378_3883) still exists.
Even after "uPnP Service" under "WAN" is disabled for sure, "Router Security Assessment" under "AiProtection" says that uPnP service still is NOT disabled.
This is because the tool checks the uPNP status for both WAN0 and WAN1. If you only use single WAN, then the WAN1 will still be set to enabled.
To disable it, enable dual wan, then set UPNP to "disabled", and switch back to single WAN.
2. "Enable Load Balance" option is now available in "Smart Connect Rule" settings under "Network Tools". I don't know what it does exactly, but I hope it's actually does what its name suggests.
That option is only for the RT-AC3200, for its Smart Connect feature that's hardware-specific to the RT-AC3200.