Your watchdog circuit is OK. I think you can leave it alone. Here is another screenshot I took again from my SPICE simulation of the watchdog circuit. I simulated normal presence of ICV pulses coming from µC pin 5, followed by a sudden stop of these pulses at 300ms. Signal in green is what appears on pin 6 of LM139. Signal in blue is RST line. Small green saw tooth signal on left is when normal ICV signal (from µC pin 5) is present. RST line remains low, situation normal. Then when ICV pulses stop being received by the watchdog, large saw tooth signal similar to what you see on your not OK unit begins to appear. That is the result of the watchdog circuit entering a different oscillation mode due to the absence of ICV pulses. That allows it to send RST pulses every 30 ms or so.
