EDIT this was not really a bug, I was looking at the wrong bits. But other things were clarified.
Or is it a datasheet bug?
SYSCFG0 bit 3 RSTPINCFG reads as 1 on the ATmega4809 Xplained Pro (0xF5) which means that the pin (PF6) is used as RESET.
However Studio says that it is set as I/O (0??) pin and in fact the reset button of the Xplained Pro doesn't work.