Did your redo 2019 by doing a NEW tax return or did you just change the one you had? You must have answered a follow up question wrong and got a smaller taxable amount. Maybe this year you answered wrong and got too much taxable? Better check the prior years too and see what was reported for the taxable amount.
The reason line 4c is now zero is because line 4d is the full amount. If they are the same then 4c isn't listed. If you still have the original 2019 file you can save it with another name (so you don't mess up the original) and look at your entries and open the 1099R form and check all the items and compare it to the redone 2019 form to see why you got a low taxable amount. Same for 2020, switch to FORMS and look around.
@dmertz
