PIR Sensor triggering voltage not enough
If the sensor is putting out 0 to 3.3 V when open circuit, and 0 to 290 mV when connected to a microcontroller, then the micro is obviously loading the signal.

A normal CMOS input wouldn't load a signal like that, so obviously that input of the micro is not acting like a normal CMOS input.  Possible reasons include:

The pin is not configured as a input in firmware.  Check your code.

The pin is dedicated to some other function.  Check the datasheet.

Something else is loading the pin.  Check the schematic and your wiring.