This is an issues I'v e seen across several versions and expansions.
Let's say I have 6 Human colonies and 2 Securan colonies in my empire. All colonies are at their population cap and the popluations are made up solely of their respective race types. Planet quality is a mix, but the Securan worlds aren't necessarily any higher quality than the Human colonies. All colonies have the same tax rate. I also have 2 or 3 resort bases on various gold and dilithium crystal asteroids.
Now, over time I begin to notice that my Securan colonies are suffering a decrease of the Securan population and the appearance of a Human population. Over time, this Human population continues to increase, and the Securan population decreases. Left to it, the Human population eventually supplants the Securan population, leaving the latter at the bare minimum 1M on the world.
Just to be clear, in my example I use Securans, but this happens to any alternate race in my empire. This is intolerable. I mean, with Securans you're not really losing any race abilities, but for most other races I would be. Maybe the migration is scheduled in the brief gaps of time between a tourist transport picking up Securans to visit the resort base and the population growing to fill the void, but it shouldn't happen regardless.
1. A migrant transport that arrives at a planet should not be able to drop off its load of people if there isn't enough room to drop them off without going over the population cap. If there is no other choice but to drop them off, they should be the ones to vanish, not the population that's already there.
2. Migration should not be queued to a planet at or near its population cap in the first place so as to avoid this problem before it begins.
3. I don't know if or when tourists ever return to their home planet, but their departure should not queue the rest of the empire to migrate to the world or for the colony in question to re-populate after their departure.
I mean, I dig that we're talking about numbers representing virtual people here, but it damages the suspension of disbelief when I have to consider that those folks going on vacation are losing their homes either to their neighbor's kids or to squatters from other worlds in the empire. Once they're there, I can't even get rid of them short of using the editor, but I can't get rid of the last 1M at all, leaving the door open for more migration and population growth at the expense of the native population. This really bothers me. Maybe it shouldn't, but it does.