328PB bootloader problem (Oscilloscope screenshots attached)

Hello,

I am having trouble installing the Arduino bootloader on a ATM328PB.
I successfully did that a few months back (same project, slightly larger PCB) but this time around I am stuck.

I am using an Arduino UNO as the ISP.

I have the following connected:

MOSI → 15
MISO → 16
SCK → 17
RST → 29

as well as:

VCC → 4, 6, 18
GND → 3, 5, 21
XTAL → 7, 8
GND → 20 (via 0.1uF Cap)
VCC → 29 (via 10K Resistor)

I already have tried repeatedly with different chips and verified every single connection. I even managed to hook up an oscilloscope (see attachments).

As you can see, MOSI seems normal, as does CLK. MISO, on the other hand, presents some activity but it also is extremely weak (around 300mV pp).

Would you be able to suggest where I should look next in order to solve my problem?

Thank you!
Alain

After posting here I remembered I had an USBavr and tried with it. It works.
That's when I realized that the 328PB is NOT the same at the 328P! (signatures are different) Bummer!

Anyway, even with the USBavr, the MISO signal is still a lot weaker than the others and I can't understand why.

You could post the schematic and PCB layout perhaps ?

Here are the schematics of what has been populated so far and the pcb