That is pretty typical ... they are designed that way to prevent entrapment. Of course whether you have flow through there or not is dependent on your plumbing and valving.
i just tested my own main drain over this same concern. I thought perhaps the skimmer/main drain ratio valve was installed wrong. But after moving it full one way and putting my hand in the bottom of the skimmer, under the venturi return, and then full the other way and having no suction in the skimmer, it's clear the valve is correct. I also folded up a bouganvillea leaf (no chance I spelled that right) and shoved it through the main drain grid. Got out of the pool and looked in the filter that is just before the pump and the leaf was in there.
However, I can see a bunch of sediment accumulated in the ring of the main drain; the suction isn't strong enough to actually carry it around into the input ports of the drain. I'll probably have to get a vac attachment down there every once in a while to clean it out.
Other ways to test it are: swim down with a bottle of food coloring and see if it gets sucked in. Or switch to full main drain suction and set a rubber mat or something over it and see if it starves the pump and the return pressure drops to nothing.
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