It is showing differing rates, but that isn't so unusual when the total number of days changes. When it's booked 8am to 8am, using a random example of the smallest car, it is showing as 35.40/day...add on 1,2,3, or 4 hours to return time, and it shows an extra day to the total and the price is now shown as 30.35/day (both pre taxes). Rate change shown the same way for all car classes.

It's not the dollar amount, it's the principle of it, as it is a published benefit, and the 4 hours is genuinely useful here, as otherwise we have to either wait around for several hours after arriving to pick up (then again it is air france so odds on we will be late), or we get there for departing flight several hours earlier than normal and needed (carry on only both ways).

I've also never had an issue booking anywhere with it before - and just made two more bookings in the US for the holiday season where it worked as normal - so this is odd.

Edit: it does it with a dummy booking for CDG and NCE too! Is this a France specific thing?
Edit 2: Also does it at MAD, BER, and LHR. What is going on?