P Channel MOSFET as a Load Switch
There's nothing inherently wrong with your schematic. I would omit the NPN however and just invert the sense of the MCU output (you can do that because your MCU can swing to 3.3V and can fully turn the FET off.)

You could have a current loop through your board that's dragging down your 3.3V. Try wiring the FET source directly to a separate power supply to see if that's the case. If so, review your power supply design and layout carefully, and try to separate the high-current paths from the MCU rails.