Those gaskets do leak from time to time. I've had to replace them more than once. However, if your handle does not go to the up position by itself it may be either the "O" rings on the stem or the spring. There are two O rings on the stem and if they get old and hard they can crack and prevent the seat from going all the way down to seal off against the gasket. The spring can also lose its spring and needs to be replaced from time to time.
The O rings and gasket need to have a very thin layer of a silicone grease applied to the outer edges so they will slide and seal properly.
So if you suspect a leak from the multiport valve I would replace the spider gasket, both O rings, and the spring. It's not very expensive to do the whole rebuild and that should correct the problem.