In many other JetBrain IDEs (like PyCharm and IntelliJ), you can set different compiler/interpreter for each specific project. This preference will be stored in the project and you'll get the correct toolchain chosen next time you open up the project.
In CLion this seems to be missing. You have to set a global default toolchain and it's used by every single project. If you set the default in one project and then open another project, the other project's toolchain will also change.