Gonna bump this up since I'm seeing the same thing and a cursory glance through the search feature isn't turning up anything definitive.

Had a UP resolution that should have passed easily (the same one as the OP, ironically enuf [though it has been changed a bit in the meantime so the values aren't the same]), but the Drengin defied the vote.  Sure enough, they are on the list of "Defiant Races".  However, the measure didn't pass, even though it had more than enough votes to do so.

So is it in fact a situation where if one faction decides to say "Screw You, I'm Outta Here". the measure automatically fails?  Seems a bit OP, especially since the Defiant Races won't be covered by the UP.  Doubly especially so as the number of factions is increased, as it would only take one cranky AI to bring the whole house of cards down each and every vote.

Is this in fact Working as Intended, or is it a bug?

No, it works.  I went to retest it and I had an Autosave set right as the vote was taking place (the game auto saves and then the Start of Turn stuff happens - this time, a UP vote was first in the queue).  Reloaded it and it passed, this time with the Drengin sticking around.

In fact, I couldn't get them to defy the vote after three or four attempts to replicate the bug. Don't know what the percentage is for an AI to turn into to Captain Cranky Pants though.

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As an aside, even though the measure passed, I'm not being taxed for it.   I should be paying a 3% tax for it, but I'm paying nada.  Checked the Economy Screen as well,  and it's not showing up there either.  I AM getting the benefit for it, however.