Short-term hyperinflation driven by the basic income driving price growth. That's an interesting thought, and one that I've seen come up a few times in conversations around basic income (in fact, it's very similar to the model inflation explanations given in econ 101, especially when taught by neo-classicists).

The model suffers from extreme over-simplification, in making two key assumptions.

1. It assumes that all recipients seek the same goods, when in fact they seek different goods. The advantage to basic income programs over other handouts is that they allow recipients to identify the best way to spend the money, instead of it being decided arbitrarily by a bureaucrat barely aware of their situation.
2. It assumes that even when assumption 1 doesn't hold, the basic income spending is a high proportion of economic activity. (Similar to your base case where basic income spending is 100% of economic activity.)

Basic income programs would work better (and be more sustainable) if they explicitly target a low percentage of economic activity. Instead of saying $xxx per month, you target xx% of GDP (based on a rolling three years to smooth business cycle shocks). The intention is not for it to be somebody's entire income, but to be a base that insulates them from economic shocks in their own life. (Like how car insurance means that an accident doesn't destroy your finances.)

In this way, basic income has a high impact on the people that need it most, and a much lower impact on people that don't need the money at all. In fact, at a certain point, the taxes on a person's earned income would exceed the basic income they receive, meaning that person does not receive a basic income (in net), but it would still be there if their circumstances changed and they became more dependent on it.

The 'income multiplier' in economics measures how much each marginal dollar of spending impacts economic activity. The multiplier is inversely correlated to income levels. (The more somebody earns, the less each additional dollar that person receives will boost overall economic activity.)
So a basic income that removes excess income from high earners (with low multipliers) and passes it to low earners (with high multipliers) actually increases overall economic activity. There has even been recent research positing that the slow economic growth in the current 'recovery' cycle in developed nations is a direct consequence of our extreme wealth distributions.

Essentially, I'm saying that instead of creating inflation in the goods needed by basic income recipients, it would actually increase overall economic activity; maximizing multiplier effect by increasing the incomes of the demographics with the highest multipliers.
For the record, I don't advocate command and control economies, and I see serious drawbacks to taxation-driven economics. That's why I designed SBI as a social experiment to incentivise people with high incomes (dolphins & whales) to sponsor quality content creators of their choice for a basic level of support.

The question the experiment seeks to answer is 'Can we create a meaningful basic income program that is driven by voluntary donation/investment instead of command and control?'

While there have been some criticisms about the ROI of SBI, particularly from people that do not see the social vision and think of it merely as a money-making scheme, ROI was never the main point. The main point is bringing as many people in as possible without them having to pay anything at all. This is one reason why we almost never include ROI in our updates.

Given that more than 60% of members have never paid a single steem-cent to SBI and still receive their 'basic' upvotes, I would say that it's working exactly as intended.