The problem with a mixed ethnic society is that even when everyone is getting along, when times are tough, people can revert back to clans and ethnic identity. It stops being 'all for one' if one group thinks another is getting an edge, which is always a mathematical certainty over time.
Its the ethnic part, and maybe thats why some promote mixing. Because it can only work if in fact you know what it is. Its hard to point to ethnic identity if it has no name or you don't know what it is.
So forget any emerging visible trait. Lets say a group that is totally mixed has an identifiable birthmark that looks like a flower on their neck. They eventually work hard, gain wealth and wisdom repeated in their internal lore and begin to be prosperous enough to be a recognizable group; call them 'flowers'.
Over time, they disperse, intermarry and eventually leave their roots of centuries earlier. But they still have a visible mark and can be identified.
One day, the nation they are in starts to fall on hard times, and for any number of reasons, they fall behind. Eventually they notice a pattern that could be for any number of reasons, reliably shows that 'flowers' are getting the short end of the stick. The reasons are complicated, but the outcomes is predictable with high probability.
So the reason doesn't need to be understood, its complex. But the rule is consistent, 'flowers' are getting screwed and others are benefiting from the same general dynamic.
At that point, 'flowers' begin sharing notes and noticing the pattern and unite to find a solution. The group recognize each other by the baked in birthmark, and they begin to commiserate on the situation, and recall their lore and past glory.
At which point they decide its everyone against them, and behave that way.
Its a self defense feature baked into ethnic identity. Its not wrong. Its just a problem thats going to predictably emerge in any multi ethnic society.
https://www.youtube.com/watch?v=goePYJ74Ydg