there appears to be a bug with that option, as it smooths out totally unconnected parts and the model looses overall detail. Should be reproducable, is here at least.
Yes, I am getting this too in V3.1 Very annoying as I would like to add detail to a section of the mesh but it smooths out.
a workaround for this is:
- At lowest level hide what you want deleted.
- Go to your highest level.
- Hit the Delete Lower button.
- Now you can delete your hidden parts.
- Reconstruct Subdivisions, you may have to Enable UV’s if you don’t already have them on.
An easier way to retain all the detail when deleting hidden geometry is to use masking.
1) Hide the geometry you want to delete at subdivision level 1.
2) Apply a 100% mask to the visible geometry (Ctrl + A).
3) Go to the highest subdivision level to register the mask on all vertices.
4) Return to the lowest subdivision level and press delete hidden.
Wow TVeyes thanks for that, never thought of doing it that way, just figured it would mess up regardless of the masking.
I never thought of doing it that way either 
Unfortunately I cannot remember who came up with this particular trick a couple of years ago.
Cool, thanks for that TVeyes, I would never have got there by myself. It works perfectly.