Plunger in my bidet is releasing water

Do you mean it has a "push up" waste fitting, and the weight of the linkage and plunger is causing it open slightly?

Many of these can be adjusted easily. If you look at the waste linkage, there is normally a vertical bar on the end of the plunge lever, and that connects via a link block to the horizontal bar that enters the side of the fitting at the bottom of the basin, and acts as a leaver to raise of lower the plug.

It normally passes through a nylon ball retained in a socket via a knurled tightening cap. This not only keeps the connection water tight, bit also allows you to adjust the amount of friction that keeps the leaver in the desired position. Simply tighten the cap a bit for more friction.

See around 1:10 in this video: