I have just had the weirdest thing happen over the course of a year or two in my current game. I'm about 80 years in and my civilian ships must have exploded in quantity.
At some point in my current year, I must have clicked that Mars (450M people) and Luna (500M people) could be the source of colonists. I didn't notice this happening, but Mars was completely depopulated. When I checked, it had ZERO population. All of its infrastructure was fine. The population was completely gone.
I reloaded a backup game, turned source off for both Mars and Luna, and then within three months, I saw that Earth had lost about 100M people. So then I had to turn Earth off as a source (for the first time all game).
Is this common? Is the civilian AI really stripping my colonies of people at this rate?