There is, no joke, a surprising amount of libertarian support.
The two most prominent libertarian supporters of a UBI are Matt Zwolinski, a libertarian philosophy professor at the University of San Diego, and the economist Michael Munger.
There are a whole bunch of different ideologies in that quadrant of the political compass (for lack of a better visual aid), and anarchism is only one of them.
Yeah, it does make me nervous because it feels like a lot of libertarian ideology around UBI involves getting rid of social security, food stamps, etc. etc. without guaranteeing UBI will cover basic survival.