UBI is implemented tomorrow. Every citizen gets $1000 per month.
Landlord now knows you have an extra $1000 that you never had before. Why wouldn’t the landlord raise prices?
Now you have an extra $1000 a month and instead of eating rice and beans for a few meals you go out to a restaurant. The restaurant owners know everyone is eating out more so why not raise prices and maximize shareholder profit as always. The restaurant/corporation is on TV saying, “well, demand increased and it is a simple Economic principle that prices had to increase. There’s nothing we can do about it”.
Your state/country has toll roads. The state needs money for its deficit. UBI is implemented and the state/country sees it as the perfect time to incrementally raise toll prices.
Next thing you know UBI is effectively gone because everything costs more and billionaires keep hitting higher and higher all time net worth records.
If you give everybody money, demand will not increase across the board. Demand of necessary products will go up while demand for luxury goods will go down, as those in more need will be doing a higher proportion of the spending than before. This will allocate more resources to necessary services instead of luxury goods. This won’t increase pricing indiscriminately across the board–it probably won’t even increase prices at all if the financing comes from taxes or some other “real” source.