It's not the FET conducting.
If you turn off the FET, you have an unpowered sensor or potentially multiple unpowered sensors on SPI bus.
If any pin is set high from the MCU to the unpowered chips, like SCK or MOSI, and especially the SS, it will try to pull the switched 3V3 supply high to 3V3, through the pull-up and via the internal ESD protection diodes of the sensor.
You can verify this be removing the FET so it can't power the rest of the chips and measuring the switched supply.
(As a side note, even if it is a problem of the FET conducting, if you fix that issue, you are still left with the issue I said, and it must be fixed. Also you cannot have unpowered sensors on a bus if you still expect to be able to communicate with other powered chips on the same bus).