[Suicide Junkie]

Well, I know it was fixed somewhere, and that was the entry in History.txt that looked most like it.

And it works fine for me in V1.49.
954M people just colonized a 1000M max breathable.

The problem was that worlds were treated as domed until some population got added. In the case above, a domed planet of that size would hold only 200M. The bug caused the colonizer to drop only 200M, instead of up to 1000M.

If your colonizer holds way more people than your planet can hold, even undomed, of course the excess people will die.
It is entirely possible to build a Baseship that holds many Billions of people. If you then decide to colonize a tiny moon with them, don't be surprised that they don't all fit!