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This is normal Python behaviour.
When you import a submodule, that submodule is set as an attribute on the parent module. In this case, when simplelib.models is imported, the models submodule is set on the parent module simplelib. The parent module namespace is the same as that module’s __init__.py global namespace. This will overwrite the old value.
If you put simplelib as the first entry in INSTALLED_APPS, its models submodule will be the first models module imported by Django. This will replace the simplelib.models attribute before any model fires the class_prepared signal. On the other hand, if you put simplelib at the end of INSTALLED_APPS, Django will load simpellib.models as the last models module. Any models that are imported before that will fire the class_prepared signal before simplelib.models is imported, and before the models attribute is replaced with the submodule.