While what you say might be true to some extent, the upper-middle class will then have the additional UBI income with which to spend on buying property from tho

While what you say might be true to some extent, the upper-middle class will then have the additional UBI income with which to spend on buying property from those who are selling it. So while there may be less property hoarding on the extreme end and some selloffs, the drop caused by an increase in supply will be absorbed by an increase in what people are willing and able to pay in competitive markets.