I found that for several fonts (e.g., Fira, Chalkboard, Chalkduster) the pdf export doesn’t work correctly with some standard math symbols. It seems like the corresponding virtual characters sometimes do not work as expected. There are no such problems with the standard Roman font and Stix.
Below you can see an example of such pdf export. I tested it with several pdf viewers with the same result:
This output corresponds to the following TeXmacs equation with Fira font as the document font:
P.S. I wanted to attach the corresponding TeXmacs files and the produced pdf files but didn’t find how to do it in this forum.