A real head-scratcher here...
I use a style in much of my work that makes use of the "archstyl.shx" font, which is a hand-lettering font not standard in Autocad. I depend on this font extensively, due to its unique way of displaying fractions. The font file exists in all my search paths, and never had I had a problem with it, except in this one particular project, whose master DWG and all XREF files were provided by my client.
In the master file, and in any of the xref files, any text I create using this font appears as simple block letters. Additionally, any block I insert from my library containing this font appears incorrectly. I have searched for any font or styles of the same name in the client-supplied files which might be overriding my own, but find none. Once I have inserted a block, or created a text in the suspect files, the proper style name and archstyl.shx font do appear in the Styles dialog, so I know Autocad is finding it, but all attempts to get it to display properly have failed.
I have tried creating new and unique styles within the drawings, using the archstyl.shx font, with the same results. Remember that this behavior appears ONLY in THIS particular group of DWG files, so I'm thinking there must be some setting within those files, and saved with them, that is causing this. Other styles and fonts within the same drawing group appear as they should.
One other thing... This is not confined to Autocad 2019, as I have run the files on a 2018 install, and it does the same thing.
Anyone have any ideas? I've burned out too many brain cells trying to get my head around this one...