February 13, 2018 UPDATE

I've discovered (realized, really) an NFS issue. However this is not
specific to Mach386.

Because I am mounting mammoth-sized NFS filesystems currently, while I
look for and recover additional Mach386 data from my old media, I began
encountering segfaults and other strange behaviour in some programs.
I just realized that this is due to the filesystem itself. Many
programs simply aren't able to work with the filesystem, particularly
when creating temporary files... Like ex/vi does.

Lesson re-learned.