One of the classic unintended consequences of social welfare is making someone at the bottom unwilling to work. We saw this during the pandemic when people in formerly low-wage jobs got a lot of cash assistance and stopped being interested in low-wage jobs. (Remember all the "help wanted" signs and early closing hours at local restaurants?)
I'm curious to see an example scale that would continue to incentivize social behavior the whole way up the chain - avoiding the "oh I don't want to make $100 more dollars because I'm in a sweet spot now and bad things happen at $99."
You can certainly argue that many of the current disincentives are bugs in the bureaucracy. I'd like to see a proposal for the UBI tax scale you describe that doesn't have any bugs (that is, bumps in the distribution where people are afraid to reach for state C from state A, because the intermediary state B is worse than A).