Intermittent clutch (brake) issue Freightliner Cascadia

2016 Freightliner Cascadia DD13 , Eaton Fuller 10 speed, 425k miles. Every time it rains, my clutch starts acting up. It's like my clutch brake is not functional. I have to shut off the engine to slip it into gear, start engine in gear, them proceed as normal. The mechanic at our shop checked the gaps and said they are perfect at 1/2 and 1/8 of an inch. This condition will last an hour, a day, or sometimes a few days then as sudden as it starts, it starts working perfectly again. According to the mechanic, it's a self adjusting clutch and has a wear indicator showing approximately 50% of life left. The rain part is what is particularly baffling. Any thoughts at all?