I've been trying out the new 2.92 version including megastructures. I play on a medium sized map with 8 opponents, 1 sector. This is the classic gameplay experience that I want. After a while, I noticed that there were only 4 opponents, not 8. The game somehow has "swallowed" the 4 other races and they never made it to the map.
After this has happened to me for the 2nd time, I started a new game with "Cartograph" to see the homeplanets of the other species. Yep, always only 4 opponents on the map. I tried it with only official races and changed the star density settings around a bit on a medium map, always with the same result.
Stardock, please look into this. I can post screenshots if you like.
Good luck and a happy winter time