@Kalika3 "how am I suppose to know it’s an instant book".
If you simply click on "Book Now", that is an Instant Book. Not all hosts allow IB, in which case guests have to send a booking request to the host, along with hopefully a nice message, and the host has to accept the request before your card is charged and the booking confirmed.
If you aren't really sure whether you checked the payment option you wanted, I don't see how you can be upset about it. It could also be a tech glitch. And I don't know for sure, but maybe even if you check to pay half upfront, Airbnb might not allow it on some bookings.
Don't see that you have anything to be suspicious about- you have paid for the booking in full, even though you would have preferred to make 2 payments, and have a confirmed reservation. It's not like you were charged twice for the same booking.