I am watching a tutorial(7.24 min https://www.youtube.com/watch?v=QmmSAsn_mOk) where a simple mesh is ment to be remeshed for later sculpting...
The Voxel remesher has sometimes this problem and creates a mesh with a lot of holes, flipped faces, or missing parts when the original mesh is not manifold (watertight). It might be the missing thickness.
The solution as shown in the video is to close the mesh and add a face at the top and bottom.
Alternatively, you can try the Remesh modifier with one of the first three options: Blocks, Smooth, Sharp. Apply the modifier to use the mesh in Sculpt mode.