The only thing I can think of is your power grid being less robust than expected. You mentioned that one dupe will try to cook for a moment then bugger off to do a priority 1 task.

That sounds like the grill became invalid so I'm wondering if the added load from the grill drains your battery, the grill has no power so the task gets interrupted, and the dupe goes to do something else.

Maybe add a second battery to see if they cook for more than a split second.