Figured it out.
I believe the PT100 wire gauge was too small to make a good contact in the connector.
It was good enough for resistance reading but not when things were powered up.
I seperated out the PT100 wires out and used a smaller JST plug and it's all working now.
Thanks for the patience.
@calvinx Welcome to today's world.
I'll just leave it there.
Just thought I would post to wrap up my problem and incase anyone else runs into the same issue.
I was using a Makerhive Yellow Jacket Quick Clip.
It seems like the gauge of the PT100 wires was too small. It made just enough contact for the resistance measurements, but failed when powered up.
When I routed the PT100 using a seperate JST plug, it all works correctly now.