I discovered an interesting quirk related to the IWDG.

If I program my pyboard using pydfu, then trying this in the REPL immediately afterwards hangs:

>>> wdt = machine.WDT(0, 5000)

By hangs, I mean it never returns the REPL. I added some debug prints and it seems to stuck in the IWDG->SR & 1 loop and the value of SR is 3.

If I hit the RESET button, then the wdt = machine.WDT(0, 5000) works exactly as expected.