Confirmed with 1.20.2.rc3 on a FiPy, almost always. A simple fix is to check for values <= 0, and return immediately in that case. Tested and confirmed, it works.

P.S., just in case someone from the Pycom team is taking care of issues: I added these lines to the start of the function mp_obj_t machine_sleep () in esp32/mods/modmachine.c:

    int64_t sleep_time = 1000; // Safe default value, just to silence compiler warnings
    
    if (n_args > 0) {
        sleep_time = (int64_t)mp_obj_get_int_truncated(arg[0]) * 1000;
        if (sleep_time <= 0) {
            return mp_const_none;
        }
    }

And deleted the then obsolete line to set sleep_time a few lines down the row.