I'm not a specialist in any of this either, but I thought I'd share a couple of thoughts I've had on this
I think inflation is primarily a question of supply and demand. For example, central banks in OECD countries are currently worried that inflation will go up, because (1) supply chain issues and agricultural issues (unpredictable weather due to climate chain) mean that the supply isn't there; and (2) a lot of people have cash to spare since a lot of things they usually spend money on have been unavailable during most of the pandemic. That sort of dynamic isn't affected all that much by UBI providing people scattered across the country with more buying power - especially because these are not people that can already make ends meet and will now have money to buy additional goods. UBI rather is intended to affect people that have trouble buying even the basics, and won't turn them into significantly larger spenders overnight.
Another point is that an important purpose of UBI is to lift people out of crippling poverty. One way or another, that's generally what governments and people want to see happen anyway. So if ensuring people have a minimum income required for living increases inflation for the entire country - then the choice we have is (a) leave all these people poor because we don't want inflation; or (b) deploy anti-poverty programs (UBI or anything else) and accept that they come with inflation. I don't think anyone would choose for option (a).
Finally, UBI in the long run shouldn't put a lot more money in circulation. First, UBI is part of the redistribution part of taxes. Taxes serve two purposes: pay for government spending (e.g., infrastructure, military, etc.), and redistribute money to protect the poor against the excesses of capitalism (through welfare programs). UBI is a welfare program, and should ideally be paid for by raising taxes on the rich(er). In the long run, however, UBI might largely pay for itself. Poor people on average to cost the state a lot more in terms of healthcare and related than less poor people. So lifting people out of poverty should lead to a significant reduction in government healthcare costs. Also, welfare programs tend to be complex and cost a
lot of money to administer. UBI would be a blanket program that eliminates the need for almost all other welfare programs, which should cut significantly in government bureaucracy costs. Further, early studies suggest that people on UBI take time to retool and get in a position for better jobs - i.e., better paid jobs, meaning that they get above the UBI threshold and start paying more in tax than they receive in welfare. And finally, UBI might contribute to an overall rise in bottom-level salaries, as people would no longer be forced to take the shittiest and most poorly paid jobs around just to make ends meet - forcing employers of those jobs to improve pay and conditions to be able to hire people. This will again help to lift people out of poverty, reduce their healthcare costs, and reduce their dependency on welfare.
I could also add that, in general, more equitable societies tend to have less crime overall, better health overall, etc. - which also makes the government's work easier and less expensive. But we're getting pretty far downstream here now. Still, it adds to what I think is my general point: there is a lot that UBI might affect, and most of that appears to be positive, in quite a few different ways. Given the complexity of society and general social dynamics, though, it is hard to predict how exactly that would all play outand so these pilot studies are invaluable. The only issue is that they tend to be designed on a scale that's too small and scattershot to be able to really predict the effects of a full roll-out of UBI - but I guess we have to be happy with whatever we're getting.