The particles has residual velocity so the 0 velocity at the surface means that it will keep going out slowly but surely. You need to have a small invward velocity at the surface to avoid that.
Alternatively, use a streamtracer that support boundary interaction like the LagrandianParticleTracker.
More complex to setup though: https://www.kitware.com/vtk-and-paraview-add-highly-configurable-particle-tracker/