I think it's Disco. I ported one of the Disco lots into my hood for some reason and then tried to use the mixologist as a general townie to talk to about a investigator quest. She wouldn't get down off the bar to talk to me, let alone serve bar.
I'll test this then and get back to you on my findings.
Edit: Tested the theory of the disco, no go. Same lot the sim turns up, doesn't go to the mixology bar, and instead dances, or she just leaves. I have tried replacing it, and deleting the bar and putting it back, but that just either fails to spawn a new one, or again, a dancing lazy bartender!