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.
Forget the vents, it's not them.
Is the water level in the tank proper? Have you changed the flapper valve? A bowl gets refilled partly by any water that does not drain out during the flush, but primarily by what comes out from the hose emptying into the overflow tube. If that tube has a kink, or there's crud in the line to it, or the flapper doesn't open long enough, the flush can be compromised as can the eventual water level in the bowl. Also, if you changed the flush valve, it may not have the proper balance of flow between the tank and the bowl so the bowl may need more by the time the tank is full...it's a balancing act. The older ones overfilled the bowl, but the excess went down the drain...can't do that anymore and stay within the 1.6g mandate - both the bowl and tank need to be filled at the same time and balance is critical. On yours, seems something disrupted that balance.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013