Answer
This happens in the event that the road segment does not exist in Open Street Maps (OSM).
We can still sometimes see a speed limit as it is also pulled from HERE maps; however, our edits go by the OSM layout.
Thus manual edits will fail when no OSM road segment exists.
Google/HERE Maps:OSM:
Additional Notes
Please see the following Geotab Blog article for further information about this topic.Road Speeds: How to Use and Change Them