@Jessica-and-Henry0
I've also had this problem in the past when making alternations to long-term reservations. I didn't even realise it was happening because the amounts were quite small, e.g. a guest extending for a day.
Then I had a guest book for a six month stay. She wanted to check in two days earlier than originally planned, so I altered the dates. Airbnb charged her nearly £1,500 for those extra two nights (for a room that was £38 a night)!! Of course, she was freaking out. Turns out the long-term discount for the entire six months had somehow disappeared and who knows what extra fees were added on top.
Since becoming aware of this 'glitch' in the system, I usually call CS when altering a long-term reservation to make sure the guest is charged the correct amount. You can of course try to adjust the amount from the alteration page, but you have no control over the fees and inevitably, some extra money seems to 'disappear' into Airbnb's coffers. It's not ideal...