I had that problem too with a wealth of 32 and a wealth of 11 and they choose the 11 wealth settlement although those settlements were founded at a later date and the 32 wealth one was the second.
Curious, I'll take another look at the logic.
Edit: Do all of the settlements have 0% homelessness? Immigrants will only travel to settlements with sufficient housing. Or sufficient housing under construction.
Yes it did. It had enough for around 70-100 people I built the whole settlement with the starting 17 people.
I even send in a second migrant wave from the capital after a few Years and started importing food from other settlements because the 17/34 people couldn't support all the houses I had built. It took 3-4 Years for the first real migrants to show up.
You should actually still have the save with that settlement it was the last I uploaded the settlement between the 2 mountain areas. Btw. you said you wanted to use it for performance test. Were you able to fix some of the crashes happening every few minutes?