Yep, this changed when they updated the Amex Platinum package in Germany (last year?); the restriction was not there before. I found the definition of "trip" in the terms quite weird as well.

What I do now is that my Amex is still my default card linked to my Sixt card, but when I rent a car without a hotel stay or flight then I ask them at pick-up to use my LH Visa card instead of the Amex.