Use an include path like /mnt/c/libraries/lib1/include. The C/C++ log will show "Unable to retrieve file system information for /mnt/c/libraries/lib1/include. error = -1" causing all the symbols to not be found under that folder (i.e. go to def fails). IntelliSense can still succeed, because the code to result /mnt was getting run in that case.
Use an include path like /mnt/c/libraries/lib1/include. The C/C++ log will show "Unable to retrieve file system information for /mnt/c/libraries/lib1/include. error = -1" causing all the symbols to not be found under that folder (i.e. go to def fails). IntelliSense can still succeed, because the code to result /mnt was getting run in that case.