The problem with UBI is that most versions of it would essentially liquidate social services that people would normally receive. UBI only makes sense in addition to the other parts of a comprehensive safety net (universal housing and healthcare), not as a replacement.
Although this choice is probably pretty easy mathematically, it's kind of a terrible choice to have to make, so after a while the plan changed to saying it would stack on top of some other benefits.
(But none of this matters because whatever Congress might do would be different.)