IRL;

I didn't think of the internal lube as Eric has...the first thing I thought of was a stretched/stretching Clutch Cable, but I don't know, nor can explain, how this might be so thermally dependent.

I suggest you have a look at the Throughout Fork under the car, and have a helper depress the Clutch Pedal, and compare the action when cold, to the action when hot...there should clearly be similar amount of movement...if less when hot, that's a big hint!

See also: https://www.sw-em.com/Clutch_-_cable_actuated_Notes.htm

Good Hunting!