They were issued another PIN for year 2018 tax return and I am guessing that the IRS have updated that PIN in their database and deleted the old pin for year 2017? I am quoting directly from the 2018 IRS letter that issued the PIN. It states "You'll need to use this IP PIN when filing any forms 1040 during the calendar year beginning in January." So I guessed any form 1040 filed during the year 2019 regardless of the "tax year" it belongs to. I e-filed again using the 2018 PIN for filing the year 2017 tax return.