Although it's uncommon, I recently had a valve seat drop on #6 exhaust valve. My problem came from a poor head gasket seal between #4 and #6 cylinders from the chambers being clearanced for larger exhaust valves. On #4 and #6 the exhaust valves face each other. If that area got too hot due to a poor fire ring seal it could have heated it enough for a seat to come loose. You could have a piece of seat ring hanging the valve open. Pull the head and check it out.