Syntax highlighting disappears when typing

When a project (in C, not C++) is loaded, the variables are highlighted in the correct colours. However, as soon as a key is pressed on the keyboard (i.e. when typing code), all the variable colours change to white. The only pieces of code left in colour are keywords (for, while, if etc.) and strings. This problem does not occur with C++ programs, only C. I've tried changing the CMakeLists.txt file, but nothing helps. I've also tried disabling clang-tidy (Settings - Languages & Frameworks - C/C++ - Clangd - uncheck "Use clang-tidy via clangd"), but this results in only some of the variables/parameters to be highlighted, seemingly randomly. As there's no highlighting, errors are also not being identified, even ones as simple as misspelt variables.