Frequent renter here and I’ve noticed the same thing happen multiple times with Sixt lately.
I book a standard car. At pickup, the agent tells me I actually booked something smaller. Then they “offer” me a car that’s still smaller than what I booked, park it next to the correct class, and pitch the correct one as a paid upgrade.
I pushed back this last time and the agent got defensive, even saying bad reviews could affect their pay. On top of that, fees that used to be included now magically show up at the counter.
No real hack here other than be firm or avoid Sixt altogether. They used to be solid, but something’s clearly changed.