In our limited capacity to provide advice on something we can't see or test on our own, I would be vigilant in making sure as you continue to use the truck that you do not damage the engine by keeping a very close eye on your temp gauge and/or any strange noises.
With that said, it sounds like your fan is working properly (has free play when engine is off, spins when engine is hot and up to temperature, RPM's don't rise and fall exactly with engine RPM but spins independently in terms of timing). So perhaps as Umberto said, it has something to do with the electronics.
Regardless be extra cautious.