I agree, most likely the iron cover of your thermocouple started wearing off, and now it is reading the temperature just outside of the aluminum heater block, where it is slightly colder. Hence the lower temperature.
Have you recently had to disassemble your hot end, or remove the thermocouple by chance?
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nick-foley 5
Double check the integrity of your thermocouple wires, particularly where they connect to the printhead PCB and where that PCB connects to the cable assembly. Also check that the transition from the metal housing of the thermocouple to the wire mesh sleeve of the thermocouple looks good.
I had a similar problem a few weeks ago and one of those connections was starting to come loose.
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