The reason could be one of a few things.
First off, you should be aware of what controls the height of the water in a normally functioning toilet.
It's the trap weir built into the toilet that controls the normal design level.
Regardless of the cause, except maybe one, slowly adding water should bring the level up to normal, as mentioned above. The one thing that may not allow you to bring the level up would be an internal crack, which is unlikely as there would be other symptoms.
As mentioned above, (post #4) if the small rubber refill tube wasn't inserted into the 1” overflow tube, could be the problem.
Also, related to refilling through that overflow tube , if your fill valve is leaking excessively it can take enough flow away from the refill hose.
But in either case opening of the tank cover and watching while it flushes, will quickly answer if it's related to these 2 potential problems.
So now I believe we are left with the possible problem of improper draining. If it drains too slowly, taking longer than the refill takes to refill the bowl, it would leave the level too low.
The slow drain problem could be a partial blockage somewhere or sometimes a vent problem. Vent problem being insufficient air to allow the water to drain fast enough.