I had another instance of races arriving after I've already completed a sector, however I got a second win screen this time (but no reward chests on the second win). The "extra" races is probably not a bug, but the second victory screen is.
I got a Diplomatic Win with Fringe and Human, and shortly afterward a Human planet was destroyed by a monster and Overlord (who had not been in this sector yet) took it over. Somehow they got killed (not sure if it was from a monster or Human/Fringe), and Talon appeared (also not seen previously in the sector). When I destroyed the lone Talon ship and planet, I got the "Victory music and you won the sector" popup. No extra reward chests were rewarded (though the planet did drop a bunch of components, but that's normal).
Also, I am still able to accept a colonization quest when a medium colony component is involved without actually having an open medium slot.