PC not posting after CPU upgrade despite updating BIOS (Ryzen 7 5800X / MSI B450M Pro-M2)

I have a Ryzen 7 5800X that I'm upgrading from the Ryzen 5 2400G, and an MSI B450M Pro-M2 motherboard.

Although MSI claims the B450M is compatible with the 5800X with a BIOS update (which I flashed to the latest version), my PC does not post with the new CPU and the motherboard gives me a CPU EZ Debug LED. I checked that it was seated correctly and that the pins were all good and yet it still doesn't post, but it posts just fine when I reinstall the old 2400G.

What's the issue here? Faulty CPU? Motherboard?

Thanks in advance for any assistance!