One thing that the CBO is evidently doing is assuming that the "real" tax rate is the one that existed prior to 2017 and is using that to forecast the deficit. That's a no-no, if that's what they're doing: the plan was ALWAYS to make the 2017 rates permanent, which this bill does.

It's not a no-no. It's what's required.

In 2017, when the original tax rates were scored for budget purposes, the CBO was required to calculate the effect on the budget deficit based on what the bill actually said. The bill said the tax rates were temporary, and would revert back to prior rates in 2025. So the CBO scored it that way. So only 8 years of lower tax rates were included in the calculation, instead of ten years' worth. That lowered the budget impact in the 2017 assessment.

Now, the proposed bill would change what the law says - instead of the tax rates reverting in 2025, they would be extended again. Which Congress, can certainly do, but that has a budget impact compared to what the law currently says. Regardless of whether the "plan" was to always make them permanent, that's not what the law said was going to happen back in 2017. So CBO scored the bill according to what the actual bill said back in 2017, and they're scoring the current bill today according to what the law says today.

Those are the scoring rules. One doesn't get to have it both ways. If the first bill is scored in 2017 with the tax cuts going to expire, then the second bill gets scored in 2025 with the tax cuts expiring as well. You don't get the "credit" for having the cuts be temporary in the 2017 bill score without getting the "debit" for having the cuts be temporary in the 2025 bill score.