So, say that we institute UBI, and people no longer have to work. That increases the cost of labor - you can't attract a cashier for $9.30 an hour, so you have to pay $15 an hour or whatever.
This has to be passed right back to the consumer, so prices go up. Rent goes up as well - maintaining property takes labor as well. Now, the UBI is no longer sufficient to sustain a decent life, and the poor are right back to where they started.
There are several reasons for this, the biggest is that cost of labor isn't the only input in pricing.
Basically the cost of supply constrained goods will go up, but not enough to outweigh the increased purchasing power for lower income earners.
very few economists agree that raising the minimum wage by X dollars will result in prices rising by X dollars so that the effect is non-existent
Instead, companies will move the production offshore, lay people off, make smaller packages [2], use machines, use cheaper ingredients etc. and those things create the illusion that the price is not rising. Price is the same, but the quality (of goods/services & of life in communities) is going down.
cost of labor isn't the only input in pricing
[2] https://en.wikipedia.org/wiki/Shrinkflation