Another update, removing the XC9258A33EER-G voltage regulator (powered from the PMIC) makes the power consumption drop down to 38,05 µA - now looking at why the regulator is using so much power without any peripherals and the B-series board.
Update: we confirmed that the switching regulator was the issue, why? We do not know as it is the only remaining active component and datasheet specifies a significantly lower power-draw, so we are switching it out with a low-power chip.