Mixins silently fail to apply to non-game classes

[SpaceWalkerRS]

reproduction case

The reproduction case is a stripped down version of the official template mod, with two Mixins: TitleScreenMixin (which injects into Minecraft's TitleScreen class) and GLFWMixin (which injects into LWJGL's GLFW class). Both Mixins inject into their target class' initializer to print a message: Hello from the <target class> class!. While TitleScreenMixin is applied successfully, GLFWMixin silently fails.