Why is Atmega324pb pin change interrupt not firing even though input is changing?
It turns out I misinterpreted the data sheet!

In this line in section 15.1...

"The Pin Change Interrupt Request 3 (PCI3) will trigger if any enabled PCINT[31:24] pin toggles."

I thought "PCINT[31:24]" was referring to pins 31 to 24. I missed the fact that there were pins defined as PCINT31, PCINT30, PCINT29, ..., PCINT24.
For the atmega324pb, PCINT31 is the same pin as D7, as pointed out in the comments.