We had a problem #1153 which was fixed somewhen in the past.
When I changed the code of stl-exporter to work correctly with triangles I saw exactly the same problem. Parts of the stl geometry were simply missing, especially when it was called with additional assets (clothes). I had an official version running, the fix was done after the release I loaded as far as I know. In the new version it is fixed.
For a quick workaround you can change that also for the current release, using the appended file also triangles are supported correctly:
in "plugins" directory change
9_export_stl/mh2stl.py
to my version:
(please make a copy of the original file before ...). I hope this helps ...