Had the same issue after a SSD upgrade. I’m with osx30 on this one. It was simply the heat sensor connectors not seated properly on the logic board. I blame my aging near vision. Even with good reading glasses I couldn’t see it. But when I opened up the case again and used an OptiVisor and some good light, I could clearly see that one connector wasn’t snapped all the way in and one was actually inverted. Once those issues were corrected, the fans behaved normally. All I can say is look carefully at those connectors.