That isn't a problem it's expected to solve. It's a completely independent problem that already happens whether you have a UBI or not.
The solutions there are likewise independent. Build more housing so it isn't supply constrained and doesn't increase in price to consume any gains made by the working class. Stop issuing student loans (students now have a UBI and don't need them), because student loans inflate the cost of college relative to other things which loans aren't available for. A UBI doesn't do that because the money isn't only available when spend on college.
> An actor is still a single actor, so if one actor has an over aggregation of wealth and has trouble spending it effectively the potential of that wealth is wasted when at least some actors don't have enough.
That's not how money works. Money in a bank account isn't being "wasted" because it isn't consuming resources, and if hoarding is causing currency scarcity (i.e. deflation) then additional money can easily be printed.
> So surely trillions of dollars of UBI injected directly into the hands of the poor will stimulate the economy. But for how long until the cash starts to aggregate again into slower pools of cash where throughput is limited?
For as long as they keep getting the UBI. Where do you think the money to fund it would come from?