water draining from toilet after flush
"after flushing drains to a low level"


The level in the bowl after flushing is determined by:


the length of time the flapper is held open (not open long enough = no flush, inadequate flush, or low water level in bowl after flush)


AND


the bowl-filler function of the fill valve in the tank/cistern. Commonly, the bowl-filler function is arranged such that a hose directs water into the bowl (usually via the overflow tube and rim clean holes) during the entire time the fill valve is open. Some fill valves allow adjustment of the bowl-filler duration/flow/volume.


If it were a venting problem, it would occur at random times (or specific times if related to other usage of drains), rather than only when the toilet in question were flushed...