STUSB4500: Unable to negotiate USB-C PD voltage higher than 5V
After three days of tinkering, I found out that ST is offering two products with a very similar name. I've accidentally purchased STUSB4500LQTR instead of STUSB4500QTR, which does not support USB PD, and has the voltage fixed to 5V.
Unfortunately, apart from the name, also the pinout and I2C programming interface are similar. It allows you to set the voltage and current programmatically, retains the values in NVRAM, but never uses these values or does the negotiation.
ST USB Type-C and Power Delivery Controllers: