Amending a 2020 return doesn't populate the previous refundable credits

You may not need to correct it. The amended return would only report the change. Can you please clarify your question? Are you saying that there is nothing shown on Column A Line 7 of Form 1040-X? Nothing should be in Column B or C for Line 7, but you should see the Credit listed as Originally filed.

If you do not have anything listed in Column A, revisit the stimulus payment section to verify that the input there is still the same.

1. Go to the search box and enter stimulus. Click the Jump to link.
2. Answer Yes.
3. At the next screen, Let's double-check the amount you received, answer No to edit the first-round and second-round amounts.
4. If no stimulus amount was received, enter 0. An amount must be entered in each box. When finished, click Continue.

The amended federal return can be e-filed, but only Georgia, Michigan, New York and Vermont state amendments can be e-filed. Otherwise, the amended state return must be paper filed. TurboTax will provide the directions. 

For more information, see: 

What is the IRS 1040X Form? - TurboTax Tax Tips & Videos

Line 15 column A on the 1040x doesn't have the value from line 32 of my 1040 that was filed. This is making the net change positive for line 15 on the 1040x when it should be negative in my case. Is there a way I can change Line 15 column A on the 1040x, or does TurboTax need to make an update so that value populates properly? 

Per Turbotax line 15 Column A on the 1040x will not include any recovery rebate (stimulus check) amount. Part of the reasoning is because the IRS will automatically adjust the recovery rebate amount on your 1040 if the calculation was done incorrectly so there isn't a need to amend the recovery rebate amount. But if you had a change in AGI then the 1040x would have any recovery rebate adjustments on line 15 column b and c of the 1040x.

Hi! I came across the similar situation as you. What you mean is there is no bug in the turbotax, right?

Yes, there isn't an issue with turbo tax.

The IRS wants to explain my entry on line 15 on form 1040x.  The 1040x form was returned to me. Column A is 0 but column B and C has same amount on my  original1040 line 30.  Is is ok just to manually write in hand written mark x on "other" on line 15 and specify it as "recovery rebate credit"?

Did you e-file your 1040x using Turbotax? 

I have the same problem when I e-file my 1040x using Turbotax. When I e-file my 1040x using Turbotax, it got accepted. However, I don't know whether it is returned to me in the future. 

I mailed my 1040x.