Thanks for both emails. Basil "fixed" my problem:
Apparently I have two fonts installed on my system --- "Tibetan Machine
Uni" and "Tibetan Machine Unicode", the and the former displays things
correctly while the latter does not. I'm sorry for bothering you with this,
but until I received Basil's reply, I hadn't realized that these are two
fonts...

Thanks again, and sorry for the alarm!

Mayer

On Sat, Jun 1, 2019 at 9:11 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: "Basil L. Contovounesios" <contovob@tcd.ie>
> > Date: Sat, 01 Jun 2019 19:04:43 +0100
> > Cc: 36020@debbugs.gnu.org
> >
> > FWIW, I can't seem to reproduce this on the Emacs master and emacs-26
> > branches (build info follows my signature).  Here's what I tried after
> > installing the fonts-tibetan-machine package on Debian:
>
> We don't know the Emacs version of the OP.  In addition, it could be
> due to a difference in the installed versions of the m17n-flt library.
>