IRS Notice - Electronically filed Return Missing Original Signature

Question.  Is "tax return is missing original signature" a common error with electronically filed returns?  

 

Background. I received a letter via US Mail from the IRS stating that my 2024 (not a typo) electronically filed return is missing an original signature. The letter instructed me to complete a declaration affirming that the return was mine, which I have done. I am anticipating a refund.

Does anyone in the Community know what constitutes an "original" signature on an electronically filed tax return? I was under the impression that entering my IRS-assigned PIN at the time of filing was sufficient - and necessary - for the return to be accepted. It appears that may not qualify as an "original" signature.

Thanks in advance for any helpful responses.