I vaguely recall getting similar issues when I was developing a syntax highlighting package for use with VS Code. It could be that some files are wrong, missing or corrupt, have wrong line-endings or the like. As a package I'm not sure why something might not be right.
Trying Code-OSS from headmelted, installed some time ago, it seems to be working for me, highlighting C and Python code differently. Though does keep "recommending" for Python even though installed and being used
It may perhaps depend on how you installed Code-OSS. This is what I used -
viewtopic.php?t=191342
If you cannot resolve the issue you may have to contact headmelted or Microsoft as it likely isn't a Pi-related issue per se. What OS and Code-OSS version do you have ? Under Raspbian Stretch, Pi 3B, Help->About for me shows -
Version 1.14.0
Commit c5e3....
Date 2017-06-09....
Shell 1.6.6
Renderer 56.0.2924.87
Node 7.4.0