Symbols not visible

[ship51]

I recently observed that Sumatra was not displaying the symbols such as +, -,*,/ ,( ,) , epsilon, etc. when I looked up the actual book of the eBook I had.
It seems it recognizes it is some character but is unable to read it. Any suggestions?

eBook

Actual book

[kjk]

Does it happen in pre-release?

If yes, please open an issue and provide a document showing a problem.

There's nothing I can do or say without access to file that shows the problem.