A sink that slowly empties around a closed stopper may have a stopper-fit or sealing issue even when the disposer itself works. Check the stopper's intended water-holding position in the model instructions.
What the observation tells you
Some accessories have separate draining, operating and closed positions. Food on the accessible sealing edge or the wrong replacement part can prevent a proper seal. Do not force the stopper deeper or add improvised weights, especially if it also operates a batch-feed control.
A useful next step
Clean only the approved accessible surfaces with the appliance safely isolated. If the correct undamaged stopper still will not hold water, request the appropriate replacement or assessment.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.