A distribution from an IRA is not an RMD. The RMD (Required Minimum Distribution) is the amount that you are required to take for the year. You can take distributions that are more or less than the RMD, but the total amount that you take during the year has to be at least the RMD amount.
On your tax return you report the distribution that you actually took, even if it's less than the RMD that you were required to take. If you take an additional distribution in a subsequent year to make up for not having taken the minimum amount, you report that in the year that you take it, even if it's for an earlier year. You do not amend your tax return for the year that you took too little.
If Form 1099-R shows the amount that you actually took, then it is correct. You do not get a corrected 1099-R if you should have taken a larger amount. The 1099-R for any year shows the total amount that you actually took in that year. Your 1099-R for the year that you took the additional distribution will show the total amount that you took that year, including both the amount for the current year and the additional make-up amount for the earlier year.
You can take as much or as little as you want. The brokerage firm is not responsible for making sure that you take the correct RMD. There are situations where you could satisfy your RMD by taking a distribution from an account at another brokerage. You could also take your RMD in several separate distributions instead of all at once. So the brokerage is not going to say anything if you take a distribution that is less than your RMD.
You can ask the brokerage to calculate the RMD for you each year, to make sure that the calculation is correct.
For the year that you took less than the RMD, you have to file Form 5329 to request a waiver of the penalty. It should have been included in your tax return for that year, but you can file it separately now. See Form 5329 and the Instructions for Form 5329 for the year that you did not take the minimum amount. You do not amend your tax return for the year that you took less than the RMD.
