[XeTeX] mktexfmt fail on non-letter

[Julian Bradfield]

I just ran xetex for the first time on my work desktop, which is
running RHEL7 Linux, with a texlive 2018 distribution.

Making the .fmt file failed owing to numbers in hyphenation patterns
in three languages: for example:

....
(/opt/texlive/2018/texmf-dist/tex/generic/hyph-utf8/loadhyph/loadhyph-cu.tex
UTF-8 Church Slavonic hyphenation patterns
(/opt/texlive/2018/texmf-dist/tex/generic/hyph-utf8/patterns/tex/hyph-cu.tex
! Nonletter.
l.41 .а҆кта́7ꙋ
              
? 
! Emergency stop.
l.41 .а҆кта́7ꙋ
              
No pages of output.
Transcript written on xelatex.log.



Presumably this doesn't happen to other people, so what could be
causing it? Checking the log, it's loading all files from the right
place - no shadow files anywhere.

For the moment, I've solved it by shadowing the three offending
hypenation files with empty files, but I'd like to understand!