Hi James,
Repost with better title. 
I use lubuntu-18.10.
Sometimes, a letter gets corrupted but I only see it in libreoffice.
This example the 'p' displays as a ',':
http://lockie.ca/lubuntu_/libreoffice.jpg
I've also seen it when '2' displays garbled.
Any ideas about how to track down what software has the problem?
It seems you're using the Lubuntu (ie, Ubuntu) LibreOffice version, which is different from the "official" TDF LibreOffice one. At times, the packaged versions within Linux distributions may introduce unwanted effects that the official version doesn't suffer of.
I'd first encourage you to download and install the said official version and test with it. If you get the same problems, come back and report here.
Dowload link:
https://www.libreoffice.org/download/download/
How-to install:
https://wiki.documentfoundation.org/Documentation/Install/Linux
(look at the Debian / Ubuntu / Mint section)
Best regards,