Simply put, inflation is a multi-variable equation. By increasing money velocity for the bottom 60-80% through a tax and transfer of existing money from the top 20-40%, a range of effects will occur that all depend on other factors. To assume inflation across the board, especially to the point of eroding the entire value of the UBI is just about as dumb as saying if you jump really high, you can land on the Moon. Theoretically speaking, that's possible, but it's really really stupid.

Some prices will go up, some will go down. Some supply will not be able to be increased as much or as quickly as others. Some supply is infinite. More money in the hands of more consumers will drive the economy, creating more jobs, and also improving the price mechanism. Because of unconditional income, more people will also be able to engage in the sharing economy as prosumers, where money doesn't actually exchange hands at all.

There's a lot to think about with UBI. Don't think it's as simple as going, "Duh, more money equals higher prices equals pointless."