i recently found that "follow symbol under cursor" to switch between declaration and implementation is not working very reliable in the default clangd setting of "try both" for "header/source switch mode". in many cases it simply does not navigate from function declaration to function definition.
This problem goes away when changing the configuration to "use clangd only".

it seems that "try both" works better when both source and header file have been open, but in the case of opening a (large) project like qt, opening a header and "follow symbol under cursor", the definition is typically not opened. it smells like a regression, as i don't remember having problems with this before, unless the project is configured with unity builds