I have an inexpensive AquaSource toilet that's about two years old that will not hold it's water level in the toilet bowl. I just did a test: flushed toilet, marked water level in toilet bowl with pencil, and observed water. Within seconds after the water shut off the level started dropping. It dropped about 1/2". Why? It needs the extra water for a good flush. What needs to be done so it doesn't drain out this water?
Assuming that the water does not continue to drain after it reaches the half-inch drop, I believe that what you're seeing is by design (intentional or otherwise). When the tank on the toilet refills, a small portion of that water goes to the toilet bowl to refill it. The bowl is typically overfilled to ensure the water level fills up to the edge of the siphon (I'm going to refer to it as such, even though this is not the name on many newer toilets) to prevent sewer gas leakage, and the excess drains over the edge. I suspect that your toilet is of such a design that the water level inside the siphon equalizes slower than one would expect.
There are a number of reasons that the water level could equalize more slowly; some models create a back-pressure inside the siphon, some create a positive air pocket for additional suction during the flush, or it could just be the intentional design of the siphon.
I doubt it is something to worry about.