I haven’t gone underground yet, but I remember reading a few posts with underground issues.
I don’t think what your doing is the problem, more that it hasn’t been sorted out yet.
I know The Fort Underground Volume lets cars drive under the landscape, but I was only hoping it would assist the navmesh.
submit a Bug Report and hopefully they’ll use your data for improvement.
Hopefully someone who knows about this can post, otherwise you may have to create a workaround.
I wonder if creating another new landscape for the mountain would behave the same way,