@Joy298 I'm as baffled as you are.... but I think it might be common.
We had the same situation happen to us recently. We were expecting 1 guest to arrive, and 2 turned up. We don't charge extra for a 2nd guest, but I wanted to change the booking so that the 2nd guest was registered with Airbnb.
Like you, both the guest and I tried changing the booking details, but it wouldn't accept as the booking had already started.... so I too, phoned Airbnb Sydney directly..... and, They couldn't change it either!!!
The support person I was talking to referred it to their 'manager' and they said that 'they'd made a note' that a 2nd guest was staying. Not very professional in my opinion either!
I do like what @Branka-and-Silvia0 suggested about getting confirmation from the guest via the message thread though..... I'll do that next time!