It should also be noted that inflation is not immediately present following a cash stimulus. It takes time for inflation to set in as a response to market conditions and as such inflation is measured as an adjustable change over a given period of time. That said specific single time examples of UBI not adjusting inflation only demonstrates a faulty understanding of economics.
Where do you think government's money come from? Financing UBI will require raising taxes.
empirically, however, things are sticky and so we have not observed concomitant decreases in employment with (small to moderate) increases in the minimum wage. my other guess (not sure if this is supported by the literature) this can also be because not only is the market sticky, but because workers were being paid a lower wage than their employer could have afforded.
We've seen this with higher minimum wages. The amount of inflation is much lower than than the wage increase, and poor people end up with more buying power.