Subjugation not working correctly in v1.78?

[Sandstig]

Well, I just sent an offer for a treaty of subjugation to a very weak empire called the "Bobroba States" and when I looked at the Empires menu, it was noted that my empire was under subjugation to them. When I checked "Empire Status" just to confirm, it was also listed, that we were paying them tariffs. This goes against the statement that in a subjugation treaty the weaker empire is subjugated.

The "Bobroba States" consist of merely one planet in one system. Their only vessel appears to be a single dreadnaught. My empire has three systems with fifteen well-developed colonies. My fleet consists of two dreadnaughts and more than two hundred battleships and a token number of battle cruisers. Is this a known bug or could this merely be a fluke?