Nothing happens to an input pin when the chip goes into power down mode.
Then it's probably the PIR itself reacting to a fluctuation in voltage and resetting when the MCU goes into sleep mode. It could only be the Pin going high or the PIR resetting. The PIR enters a calibration mode for 20 seconds and during that it goes high. Gonna try a capacitor to smooth the voltage.
What are you using to wake up the chip every 8s, the watchdog or something else? Why not use a pin change interrupt to wake up the mcu on a low to high transition of the PIR output? Post your code.
I'm using the watchdog. Can't use any of the interrupts available cause I’m already using them for other events... Every single pin on the Atmega 328 is already used (some with more than one task). Gonna start fiddling with other models for the next project cause the 328 is limiting.