Why does water in the toilet bowl empty?
The water in the toilet seeks the level of the highest level of the high level of the trap, so if you raise the toilet bowl you must raise the trap the same distance.  The trap is the curly, s-shaped part of the drain between the exit tube of the bowl and the wall. The highest level of the trap follows the line of bottom of the inner circumference of the trap where the the direction of the flushing water is parallel to the trap, as it is coming out of the climb from the bottom of the trap.

It could also be a vacuum in the drain sucking the fluid back into the drain if there is not a vent pipe running vertically from the drain to the roof to equalize the presscure.

I am not a plumber nor an engineer but a do it yourself person with a little math understanding from American public schools and a very basic, traditional grasp of physical science from experience and from the instruction of a peculiarly motivated chemistry professor at Bakersfield College, Ca  USA.