With the ATmega328PB, there appears to be a discrepancy in the data sheet (and I've downloaded the latest one). On the pinout given in Figure 5-1/5-2, and also in the documentation for the ATmega328PB Xplained board, MOSI0 is PB3, MISO0 is PB4, and SCK0 is PB5. In the Serial Downloading section, 32.9, Figure 32-6 shows MOSI as PB5, MISO as PB6, and SCK as PB7.
I'm assuming that Figure 32-6 is incorrect. (?)