This has nothing to do with a UBI. What happens if the government doesn't raise the benefits amount for a food stamp program? In both cases you can either index the amount to inflation or you have the same concern, so it doesn't matter which one you use and isn't a reason not to use a UBI.

If anything it's a reason to use it, because raising the amount that everybody universally gets would be easier to pass when there are more beneficiaries. Nobody cares about the minimum wage because hardly anybody actually makes minimum wage, whereas around half (and plausibly a slight majority) of the population would be a net beneficiary with a UBI.

Index it to inflation and someone that wants to attack the program just needs to make the Bureau of Labor unable to issue accurate inflation metrics.

I think UBI would need some sort of scaling factor but you can't just hand wave it away to some other corruptible government group.