I've been living in my current house for about a year and just recently started noticing the water level in all three toilets is noticeably lower than it used to be. The float balls seem to be fine - the refill process does not shut off prematurely. The refill hose is still in the overflow tube and I can see the water coming out of it, down the tube, and into the bowl.
The issue appears to be that the bowl itself is continuing to allow water out the drain while it is being refilled. The bowls don't drain all the way; they stop when the refill process stops and the bowls stay about filled about halfway.
I read a couple places that mentioned vent issues, but I wasn't able to find specifics. It appears (from the ground) that I only have 1 vent on the roof, which I assume services all three bathrooms. In that regard, a vent issues makes sense since I am seeing the issue on all the toilets.