The goal of a UBI is to make sure people get their essentials to live. Right now, people get those essentials, one way or another (otherwise, they'd be dead; and to the extent people starve to death in the developed world, it's issues of distribution, not production or money). This makes the UBI an accounting trick: there's no actual goods not being produced that need to be produced, and it is just shifting costs from welfare, charity, family and friends, etc to the UBI program. This is not inflationary and frees up human effort to focus on higher needs than scraping together a basket of things merely to live.
A lot of the time, though, people also want some non-essential but still pretty important things covered, which is a bit trickier. In this case, there is the potential for more money to be chasing a fixed supply of goods. This will drive inflation in the short term. However, in the longer term, capital will be redeployed to capture that increased demand (while being deployed away from the desires of those taxed to fund the UBI).
This all assumes that the UBI is revenue neutral; if not, yeah, we will get a lot of inflation.