I have added NDK from a non-standard location (Conan cache) as a custom compiler by duplicating an existing NDK and changing compiler path. But a red exclamation mark appears and tooltip says "The toolchain is invalid", however nothing says what is wrong exactly, see the attached SS. The NDK itself is 100% working.

Even if I select this "invalid" NDK as compiler for a kit, it's not actually applied (instead, default NDK is used) - tested on a qmake project. But if I edit the generated Makefile by fixing compiler path, then project builds just fine.

After restarting Creator the red exclamation mark and the error tooltip are gone.