I wonder what happens when you arrive at the hotel though and tell them you have multiple contiguous nights booked on separate reservations. I've had properties combine things into one stay but I also had a situation at an HP for a 3 night stay where I needed to recheck with the front desk each morning (3 separate folios for the stay and had to get rekeyed, but kept the same room).

These have always been situations where I was combining a cert or points stay with a paid night so I've never noticed any repricing, but I wonder if you did this to lock in a lower rate, if things would reprice to a higher rate when the hotel merged the stays.