@vogelchr It's a STM32L4 and that's a good question, I'm not sure if it would back-power the die. I guess we'll find out!
In this case there's no actual battery but Vbat is run by 3V3_SB (powered any time the device is plugged in) and Vdd is run by 3V3 (which is powered only when the device is in the "on" state). I did it this way in case I wanted to preserve any state across warm reboots, but at this time I have no immediate plans to use the feature for anything.
Worst case the 3V3_SB rail has enough current handling capacity that if I end up partially back-powering the MCU it won't hurt anything.