This type:

should have an additional field Tag string for device-role tags, as mentioned in the API docs. I tried finding a single line in the codebase that demonstrates this field being read, and was once again overwhelmed by OpenStack's signature dish, spaghetti code. As for evidence, let's go with this commit that last touched the definition of the field in the API reference. It contains several API samples both for docs and tests that demonstrate the tag attribute on networks in create-server requests.