Might it be because you hadn't met the 5 night qualifying rental period? I recall seeing this when I made my booking yesterday, and was able to replicate:
If the 5-night period is not met, you won't see the double TP text, like so:
I must still be tired from New Year or something but how is the 2nd booking not valid under the 5 night rule? The car is picked up on the afternoon of 11th January and returned on the afternoon of 16th January. So that’s overnight of the 11th, 12th, 13th, 14th and 15th before returning on the 16th.