Source code project:

#include "A.h"
 
void A::Hey() { }

int main() {}

class A {
  void Hey();
}; 

[
    {
        "arguments": [
            "/usr/bin/g++", 
            "-c", 
            "-I", 
            "myincludes", 
            "A.cpp"
        ], 
        "directory": "/bugs", 
        "file": "A.cpp"
    }
]

Reproduction:

0. Install Bear.
1. With above sources, bear g++ -I myincludes A.cpp
2. Enable the plugin CompilationDatabaseProjectManager.
3. Open compile_commands.json (shown above for completeness).
4. CTRL+K and open A.cpp.
5. Tools -> C++ -> Switch Header/Source

Actual:
Nothing happens.

Expected:
A.h opens up.

Note:
To always hit the right header file, we will need to search header dirs in GCC/Clang's order, see lookup order in GCC's manual. For instance, if A.h is found in both . and -I myincludes then ./A.h should be picked up.