Milivolt is a very low voltage, so any increased resistance in any circuit powered by a milivolt source can become problematic quickly. Glad you found the problem.
You mentioned experience with high pressure steam. I'm an ex-Navy snipe; when I heard civilian boilers operating at around 100 psig referred to as high pressure boilers, I laughed. I was used to 1,200 psig, superheated to over 600 degrees F. But then I realized that 1200 psig high pressure was required for propulsion, not heating.