Terrain being suppressed underneath the deck

[CDS]

Welcome to the forum

I haven't used your version so I don't know if the terrain behaviour
is normal or not.

In my version, for any "room" created with railing or deck railing tools
the terrain will not be clipped. So I'm not sure what's happening in your case.

If you have the slab tool, you can fill in the hole. Set the "top height" to the
terrain height and change the material to match the terrain...