OK, tested how it works as a xenophobe with 100% alien slavery tolerance and various alien planets in my empire. What i found is actually worse than posted above:
1) the ai will emancipate alien slaves that work on energy, science and capital tiles (even if the output actually gets lower due to lack of happiness).
2) the ai will, on the other hand, enslave free aliens regardless of their ethos, producing significant numbers of malcontent slaves (those individualists were at least somewhat content before with propaganda broadcasts, as slaves their happiness dropped to 0%).
3) most significantly, the ai keeps enslaving, emancipating, enslaving, emancipating some of the pops again and again, for no obvious reason. A bit undecided aren´t we?
Why does the sector AI decide about this anyway? This is literally none of it´s business, even if it wasn´t bugged out.