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What you describe is exactly the bug I'm trying to report.

 

TurboTax correctly accounts for $600 deduction (confirmed by looking at forms)

During the checking process it suddenly changes it to $300 and then reports the value is incorrect

Yes, I can manually change it back to $600.

 

However, changing a value to an incorrect amount during the checking process, even if it is then flag as being incorrect, is clearly a bug.