Is the M2 slot on the Asrock X570 Phantom Gaming-ITX/TB3 hooked up to the CPU or chipset lanes?
If the primary x16 slot drops down to x8, it’s certainly the CPU.
If the x16 doesn’t drop down to x8, it’s the chipset.
That’s what my B450-I does.
Why the need for 8x, Ryzen supports 20 (available) PCIE lanes from the CPU directly?
Some boards split the primary x16 lanes to x8 for the GPU and x4 for an M.2. At least this is what ASUS does.