In this case it's not a bug. The US tax system is a pay-as-you-go system. People with W2 income have taxes withheld from each paycheck to cover this. Non-W2 taxpayers have to cover this aspect by paying quarterly taxes. If a quarterly payment doesn't cover a quarter of the yearly taxes then late payment penalties accrue until the next payment is made and so on for each quarter. So in your case your work around was to shift payments to earlier in the year which then satisfied the amount due for the quarter.
Note: You have adjusted in TurboTax, but remember the IRS has the dates and amounts of your actual payments so you may see that penalty subtracted from your refund still.
To help in the future, use your total tax liability for 2019 to figure your estimated tax for 2020. Divide that amount by 4 for your quarterly payments. If you at least pay what your tax liability was for 2019 and pay by the quarterly due dates, the penalties are waived.
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