I\'m making a new Android project, with the standard \'app\' module, as well as a library project (let\'s call it \'custom_lib\'). In the app
This is closely related to this question.
It appears that Gradle builds all referenced modules of a project in release mode. Since custom_lib is only a library, its configuration is overridden by the module that references it. I wouldn't be too concerned about the library being built with the "release" label.
You'll note that in your gradle output, your project is correctly being compiled with the debug configuration.