Mix will not recompile the app when change the "config.exs" in umbrella

Mix should recompile the project if you change any file inside “config”.

My guess is that you may be invoking a custom task that does not ask your project to compile. It is the responsibility of every Mix task to say which state they expect the application to be: compiled, loaded, etc.

If it still fails, can you push a sample project that reproduces the issue to GitHub? I will gladly take a look at it.