Clutch fan operation

The fan clutch engagement is not influenced by internal engine temperature. The internal temperature is a direct result of what the radiator and fan clutch are doing in unison. As a radiator gets restricted/older the center of the core is where this restriction normally occurs. With the center of the core restricted the fan clutch does not have heat to read off because a restriction is a cold spot in the radiator. Since the temp radiating off of the center of the core is colder than the portion of the core actually dissipating heat the fan clutch kicks in later, if at all, and the engine runs warmer due to less cooling area and a non-reacting clutch. Now that your radiator is dissipating across its entire surface your fan clutch is reacting correctly.
Randy