Hi Flo,
I had the exact situation happen to me. I also mailed my 2018 return in February 2020. I e-filed this year's return on April 14th and my return was rejected on the 16th due to the 2018 AGI not matching their records. So, I did some research. I should have known, from working at the front desk at H&R Block the past four seasons, that when you mail in a prior year return, the average time it takes that department to even look at it, is about 4-6 months. Filing a late return usually always creates some kind of problem, because your current year is being processed by a different department and without the final approval of your prior year, they have no guarantee that your calculations are correct. Any discrepancies could affect, not only your refund or amount you owe, but would also change amounts like your AGI, which is what they use to calculate this year's kicker check.
The rule is, if you filed your prior years return by mail, after mid-October, then you must put a $0 for the prior years AGI, since the amount has not been confirmed yet.
I apologize for the long answer, but I thought it would help to have an explanation. Also, I am fairly sure that it won't take long for the corrected return to get approved due to the rush to get the stimulus payments out!
I hope this helps. Best wishes for a speedy refund and stimulus payment!
