It's unlikely a TT glitch, TT core tax calcs are solid and it follows Form 2210 for penalty calculation.
To avoid underpayment penalty in 2024 you need to have paid either 100% of your 2023 tax (110% if AGI > 150k), or 90% of your 2024 tax, whichever is smaller (the 'safe harbor' amount). If you are paying estimated taxes these need to paid evenly each quarterly (safe harbor amount divided by 4) as the underpayment is calculated on a quarterly basis, just meeting (or even overpaying) the target annual number by Q4 is not enough if you didn't pay enough from Q1. If you have a large unplanned tax event like a Roth conversion and paid estimated taxes later in the year due to that, you can adopt the "Annualized Income" method on Form 2210 to show the timing and reduce/eliminate the penalty.
If you don't pay estimated taxes but have an underpayment penalty this means your withholding is too low.
So the $394 depends on your situation for 2024, what the safe harbor was for your return and what you paid in withholding or estimated taxes or when.
In TT desktop version you can drill down in Forms view to get to Form 2210 there is a lot more flexibility to get to these details at any time you are preparing your return. I'm not as familiar with online but Form 2210 may be in the PDF with "all forms and worksheets"
See Form 2210 instructions for more info
https://www.irs.gov/pub/irs-pdf/i2210.pdf
