Zen51 Confusion with Light/Fan combo

Configuration Z-Wave
1

Have a bathroom fan/light with a wall switch and 12/3 going to the fan/light. Up until now both the fan hot (red) and light hot (black) were connected to the switch. So flip switch on, both come on. Flip switch off, both go off. Kids are getting older and not leaving the light/fan on after a shower (the only light in the whole house they actually turn off) so wanted to have the option to keep the fan running if humidity > x.
Ordered a Zen51. In my head it should work perfect. Mounted up in the fan/light unit.
The red wire at the light switch is now constant hot.

Zen51:
N = neutral (same as the fan and light)
L + IN = Connected to the red wire (constant hot)
Out = goes to the Fan black wire
S1 = connected to the switched hot wire and the Light black wire

So turn on the light switch. The S1 gets power as well as the Light molex connector. Relay kicks in, fan comes on. Turn off the switch. S1 loses power, lights molex connector loses power, relay kicks out, fan goes off.

This works UNTIL I install the light bulbs. Then the relay never kicks in or out (if I turned it on via zwave) by using the wall switch.

They are LED bulbs. I know filament bulbs can have some odd behavior but didn’t expect LED to.
I could get the Zen52 and wire the lights so the Sensor wire (S1) is no longer connected to the lights directly. Though S1 and S2 would then be connected and since I don’t know the root cause, unsure if that would still cause problems.

The wall switch is a zwave one fwiw.

2

A diagram may help. Before and after.

While your setup could work, it’s not a great practice to have a line powered zwave device that loses power on a regular basis - as these devices are part of the zwave mesh - when they go away be routing problems may occur.

3

Thanks - I’ll work on a diagram and get it posted.
The zen51 is constantly powered by the red wire though and triggered by the black from the switch.

4

Never did get a diagram drawn but decided to order the Double Relay (zen52). It came today and is working exactly like I want.
The wall switch triggers S1 and S2 rather than S1 and the lights themselves. Guessing there must have been some voltage back feeding through the bulb that was confusing the relay on S1.