MSP getting powered up from GPIO
Some low voltage in the vicinity of what you are seeing is where the internal circuitry just starts to be active enough (but not yet functional) to draw meaningful current.  Likely this is related to threshold voltage of internal FETs, or junctions, or similar.  As a result, this voltage at which current draw starts to increase is where an equilibrium is reached with the tiny current sourced through the series resistor and GPIO protection diodes.

Although this voltage isn't sufficient to make the chip functional, there are a number of parts (especially more advanced SoCs that may not have brownout detectors, vs MCU's that often do) which can as a result of this get into a state from which they won't do a clean power on reset when "real" power is applied.

This situation tends to be outside of data sheet allowances; preferably you would design to avoid it happening.