Just a data point to share. Client was a US resident in 2019 and was married to a non-US alien. He filed (incorrectly) on his own using the single status.
Client got my help to apply for an ITIN for his spouse with their 2020 tax return. An ITIN was assigned, and the 2020 tax return was processed without issue.
Then I filed an amended 2019 1040 for married filing jointly in order to use the higher standard deduction and claim a lower tax/refund. Almost a year later (today), received IRS Notice saying "The taxpayer identification number assigned to ... your spouse... was issued after the return due date (including extensions). You cannot retroactively claim the married filing jointly status."
