First, in the eyes of the IRS, the tax returns filed are associated with the year it was filed.
Therefore, a reference to a 2020 filed return should refer to Tax Year 2020, filed by April 15, 2021. A tax return cannot be filed for a year that is not over yet. For example, tax year 2020 filed 4/2020 is not possible because there are 8 more months of earnings. Something filed 4/2020 cannot be for tax year 2020.
Unless you filed for Tax Year 2020 recently in 2021 and accidentally still had her listed on your return, yes, she could be a victim of identity theft. For more information, see: What to do if you suspect tax-related identity theft.
Second, her amending 2019 would not necessarily have impacted her filing in 2020 for that error code depending on what she entered for AGI. Since Tax Year 2020 is her first time to file, this number should have been 0.
What has been researched may pertain to you, not your daughter. You may want to review your Adjusted Gross Income after filing an amended return.
If your goal is to have her qualify for the first and second stimulus and receive the payment by filing for the Recovery Rebate Credit, she must meet the qualifications to no longer be your dependent.
For more information, see:
How do I claim the Recovery Rebate Credit?
Who can I claim as my dependent?
