Check the plunger on the clutch lever. Is there any free play before the plunger hits the piston?
It sounds as if the plunger is holding the piston in slightly so the piston can't move back far enough to uncover the priming port. That makes the whole system closed and as the fluid warms up and expands the only thing that can happen is the slave piston is pushed forward operating the clutch.
Remove the lever, the plunger is threaded into a barrel in the lever. There is a tiny, (1.5?mm) Allen socket grub screw in the bottom of the barrel. Loosen that with a good, accurate Allen key then heat the barrel with a butane torch to loosen the Loctite used on the threads of the plunger and screw the plunger back into the barrel half a turn. Lock up the grub screw again, re-fit the lever and see if the problem disappears. If it still slips repeat the operation.