Hold on, let's step back and look at the entire scenario:
A Watchdog signal is ONLY sent when a channel first sees the power turn on in that channel. Caveat- also needs the electronics of the TIU powered and ready to sense that power on event to be able to send the watchdog.
I don't claim to know everything about the internal structure of the new WTIU- but what I do know is it takes time for the processor and operating system (similar to the WIU structure) to boot up from first getting power via the power adapter. I don't know if there is a secondary microprocessor involved that is the "TIU" portion of WTIU, such that it can boot rapidly and send a watchdog before the WIU portion completes booting up- which again takes significant time.
So again, what I don't know is your entire scenario. Lets say you have a power strip, and that power strip is what you use to power on your system at the beginning of a session. Let's also say you are using track power transformers that turn on track power- as soon as you turn on that power strip. The old scenario of a TIU +WIU, the WIU could still be booting up but the TIU and it's processor boot within seconds of power on- so it could send out a watchdog.
Let's say you replaced all that with the new WTIU and it's power supply is plugged into the same power strip. You walk in, turn on the power strip, your transformers apply power to the track inputs, at the same time the WTIU just got power from the power adapter and possibly still booting up. This could allow track power out to the track and yet the WTIU was not even ready to send a watchdog.
Bottom line, we need a lot more details of how exactly your setup is powered and wired, is the WTIU fully powered first and booted up (it's power adapter in a constant on outlet)? When and how are you turning on track power and what source (transformers)?