Me thimks your trans-guy should go back to school and take ..... Transmissions-101 !

HEAT would contribute-to maybe .5% of glazing of clutches. SLIPPAGE or a severe overlap of the 2/3 shift is the remainder 99.5% of potential probs.

Slippage ....due to low line pressure OR worn sealing rings OR leaking rubber seals and/or excessive clutch clearances.