It is very frustrating. I had no choice in the end for a recent flight and ended up with two redemption bookings. I found that the CSR's (I had talked with three over a couple of days as I tried to argue my point) had put notes on file to say I had been told that I couldn't have a mixed booking. With a previous booking I had no issues at all.
I have now escalated by email seeking a re credit for the additional points I had to pay.