Occasionaly I run across a toilet with an internal trap leak (there still is no leak at floor). It can drain the trap seal leaving bowl low to empty. Turn off water and fill bowl from alternate source (bucket) let water “settle” note water lever, if after a while level signifigantly lowers you possibly have a trap leak.
There is another possibilty. Is this toilet properly vented? A vent eliminates the potential syphon of a trap.
Don’t overlook the suggestions of the first reply to your post either.
Good luck!
