As the title says, the device_tracker platform does not uniformly implement the entity_namespace attribute. The issue is systemic from the device_tracker class’s lacking implementation of the entity_component class, where the parsing of this value takes place. This is misrepresented in the documentation as simply requiring implementation of the entity class for this functionality (https://home-assistant.io/docs/configuration/platform_options/).
In order for device_tracker to properly handle the entity_namespace attribute, the device_tracker class (__init__.py) would need to include the entity_component class in it’s imports. However, I am unsure of how much this change might actually break by simply adding the line from homeassistant.helpers.entity_component import EntityComponent to the device_tracker class.
A pull request for this has also been submitted to the documentation to represent the current state of this implementation.
https://github.com/home-assistant/home-assistant.github.io/pull/4197