You mention you're using the recommended amount of water according to the rice cooker... have you checked, or tried, the amount of recommended water according to the rice?
Varieties or preparations of rice use different amounts of water, sometimes varying by quite a bit - I've seen them from just under one-cup-per-cup-rice, to three-cups-per-cup-rice. Your rice cooker can't change the lines to suit the rice, and may be making assumptions that don't fit the rice you're using.
Personally, we made rice a lot growing up, and I never noticed the amount of rice we made, or the amount of rice and water we started with, actually fit the rice cooker's lines.
Beyond that, you should probably try reducing water, even if your rice recommended amount of water is fairly close to the rice cooker's. You clearly have too much of it for your rice - the amount of water making your rice come out right plus the amount that gets dumped out the vent. And rice is a biological rather than processed product, like flour or tea or vegetables, where different weather, different years, and other minutely different factors in growth, production, and usage means that one batch of even the same variety of rice may end up being different from another batch - so you may need to tweak recommendations to fit what's actually going on.