Don't know your situation, this is ours for comparison. We are retired, rely on a tax accountant to help us avoid tax penalties, and we pay quarterly estimated taxes each year. At the beginning of 2020 we planned to make a very large Roth conversion which would make up most of our "income" for tax purposes. Our tax accountant calculated our estimated taxes which were then evenly split up for payment each quarter. There is a IRS form (1040-ES, Estimated Tax Worksheet) that shows the calculation basis and is used to ensure proper rules are followed. Later in 2020, we chose to make two more relatively large Roth conversions. The taxes for each were paid in the quarterly payment following the conversions. Per our tax accountant, there will be no penalty if we do it this way.
Maybe. Maybe not. There may be other factors in your case of why there is no penalty, like perhaps you met safe harbor with the original planned quarterly payments.
Let's take an example where your early conversion generated a $40K tax liability, and you didn't have safe harbor on your taxes paid last year. Then in Q3 and Q4 you do more conversions, with a $10K liability on each. No other income tax due, for simplicity.
Initially you set up to do quarterly payments of 10K/10K/10K/10K. That's fine so far, because you made even payments, and the IRS accepts it with no other information such as the date the income was realized. They assume it was spread out over the year.
Now in Q3 and Q4 you make additional conversions, and payments for them. So your quarterly payments are now 10K/10K/20K/20K. Now the IRS has an issue with this, because you didn't spread your payments evenly. They want to see 15K/15K/15K/15K, OR they want you to fill out form 2210 to show when the income was realized, and that taxes were paid accordingly each quarter.
For form 2210, you would show the first conversion in Q1, and two others in Q3 & Q4. It will say you should have made a $40K payment at the end of Q1, nothing in Q2, then $10K each in Q3 and Q4.
Either way you're short on making timely tax payments, and would likely have some penalty+interest added when you do your taxes.
This is why the safest thing is to pay your estimated taxes for the income you take in each quarter at the end of that quarter, unless you know with pretty good accuracy what your end of year tax liability would be.
That's my understanding anyway. I wouldn't blame you for being skeptical that I know better than your tax accountant, but as I said, there might be another reason why they are saying your fine with this method, and didn't bother to explain it to you.