I've repeatedly tried to book an F award on the M&M site and although there's availability, the site displays an error. To replicate it:

Try to book an F award CDG-DXB via FRA in November.
Select any available flight in F, confirm. So far so good.
On the recap page, it says that all flights are in C, although I clearly selected the FRA-DXB leg in F. It also says that the cost is 125'000 miles, so clearly F.

What's the matter?
I've tried this over a couple of days, on different computers and browsers and from differerent countries. Always the same error.