This was found digging into bug 691185. 

When a method or property is accessed which returns a type that has not already been imported through gi.repository, overrides will never be imported for that type. This can be observed using Gst.ElementFactor.make without first importing the typelib for the type being returned (GstApp.AppSrc):

>>> from gi.repository import Gst
>>> Gst.init(None)
>>> src = Gst.ElementFactory.make('appsrc', None)
>>> type(src)
gi.types.__main__.GstAppSrc

vs:

>>> from gi.repository import GstApp
>>> from gi.repository import Gst
>>> Gst.init(None)
>>> src = Gst.ElementFactory.make('appsrc', None)
>>> type(src)
gi.repository.GstApp.AppSrc

In the former case, if AppSrc has overrides, they will not be loaded. This is also happening with properties of type Enum:

>>> from gi.repository import Gst
>>> Gst.init(None)
>>> src = Gst.ElementFactory.make('appsrc', None)
>>> src.props.stream_type
<enum GST_APP_STREAM_TYPE_STREAM of type GstAppStreamType>
>>> from gi.repository import GstApp
>>> src.props.stream_type = GstApp.AppStreamType.SEEKABLE
AttributeError: type object 'GstAppStreamType' has no attribute 'SEEKABLE'

The above messes up the GstApp.AppStreamType enum and it will not contain any values. However, importing GstApp before the property access will work.