In an ideal world low earners would spend UBI money and it would circulate in the local/state/national economy in the right proportions. In reality our capitalist system has near-perfected the art of upward movement of money. We saw this happen during COVID, and there's no reason large scale UBI would be any different.
Iirc there was a pilot done in South Korea where they gave citizens of a town a kind of UBI but it was only spendable on local stores. So you couldn't use it to buy online on amazon for example.
I haven't followed up but you're right in pointing out this problem of money syphoning.
What about where the local shopkeeper spends it? Unless that money permanently carries around with it a "local only" bit, as soon as it is spent once, it's likely eventually getting sucked out to Amazon, Walmart or one of the world's other "wealth gravitational singularities".
