I can see that there is a planet to the SE of New Suzhou based on the red borders, but there is no black hex fog. If the planet had been independent I would never have known that it was there unless one of my fleets had stumbled past it.
My guess would be that hexes are getting cleared beyond the actual sensor range of ships in dark nebulas.
This bug can also occur in Beta II.
george moromisato 27 Feb 2019:
This bug should no longer happen in Era 4 maps (with the new hexagonal nebulae).