Re-seat as in just plug it out and then in again in the same PCIE Slot?
Correct.
More times than I ever care to recount, I've had a no POST (PCI/AGP) or
partial PCIe handshake, resolved by 'taking it out and putting it back in again'
Also how is it even possible that a PCIE 4.0 16x slot can run at PCIE 5.0 16x speeds? Or maybe I'm understanding the conversation wrong?
Assuming timsdf's info is true and correct, it would have to do w/ the fact that I/O (largely) originates on the SoC (from the IoD).
The First M.2 slot and First x16 slot on AM5 motherboards are almost universally connected directly to the CPU(SoC).
Ryzen 7000 and Ryzen 9000 AM5 non-APUs have full PCIe 5.0 support In-SoC.
Apparently, Gen5 cards are 'overriding' the Mobo Manufacturer's Firmware settings and directly-commanding the SoC's NBIO into Gen5 (32.0GT/s) mode.
Which, as long as the trace layout/length is w/in spec, may well run reliably @ gen5 speeds.
[Note: back in the AM4 days, *many* Gen3 A320, B350, X370, B450, and X470 motherboards had short-lived PCIe 4.0 exposure. IIRC, The B550A was a B450 board w/ firmware allowing Gen4 PCIe.
For most non 500series AM5 boards, this capability was actively removed w/in 1-2 AGESA updates from Gen4's introduction. Not every board was 'built' to be reliable in 16GT/s (4.0) operation, and it also harmed the value of B550 and X570.]
Edit:
The MSI Mag B650 Tomahawk boards use an SMT PCIe x16 CPU-connected slot, not a through-hole mount design as Gen1-4 used.
This is an unnecessary implementation for strictly Gen3 and Gen4 connectivity.
Very clearly, at some point in time *after* the PCB was designed and laid out, MSI/AMD decided that B650 boards did not need Gen5 capability.
The board is 'built' to be capable of it. Also likely: multiple 'tier' boards, share the same base PCB layout.
[Tangential: ASrock's 'new' B650 X3D board, also has Gen5 x16, IIRC]