I had the thermostat housing crack on my 05 4.0 V6 replaced it and reused the old coolant temperature sensor and now have a slight leak at the sensor but only when cold when it warms up leak goes away I’m assuming a failing O-ring has anyone ever experienced this problem?

 

Your assumption of a failed O ring is likely correct.

Although many people have had multiple thermostat housings fail in a short time with the plastic one. But since it seems to be at the sensor itself that leads to the O ring as most likely.