Who would supply these bananas? After receiving $20,000, do you think the farmer's laborers will go to work the next day for the same wage as before? The trucker? The grocery store cashier? Will the "grocery store down the street" be immune to the higher labor costs?
The gocery worker and trucker will have a better bargaining position, yes. On the other hand, they can also affort to take a lower pay because their basic needs are already covered and any additional income can be spent on luxory. So yeah they will demand better working conditions and goods will increase somewhat in price - but there is no reason to expect that this will be proportionally to the UBI amount.
