Toilet - after flushing the bowl fills with water and then in a short time the water level drops an?

A toilet has a U-shaped bend in the base so the water can seal the front part and prevent any stinky gases from escaping into the room. That is what you call "bowl fills with water". Then the drain goes into the floor where it meets a vent pipe that goes to the roof. Any gases that collect in the drain go up the vent pipe instead of pushing on the water in the bowl. Now if there is a breeze, that changes the pressure on the system and the water goes up and down, and maybe it goes up far enough to slosh over the U-shaped bend. That leaves a lower level of water.

This is a goofy thing to see happening, but as long as the toilet flushes and refills, there is no problem.