Well it may be because it was signed by the beneficiary as the preparer. Not a simple signed in the wrong spot or forgot to sign. On ours we neglected to sign the portion for my step daughter, they just rejected it without cashing the check. We signed it properly and resubmitted. Thankfully we only lost about 20 days. Wife was just approved and sent to NVC but now we are waiting for step daughters to catch up.
After reading the section it appears that preparer is probably the one spot if signed by mistake cannot have a exception or correction for good cause.
Chapter 5 - Interpreters and Preparers
If an interpreter assists the benefit requestor in reading the instructions and questions on a benefit request, the interpreter must provide his or her contact information, sign, and date the benefit request in the section indicated.
If a preparer assists the benefit requestor in completing his or her benefit request, the preparer and any other person who assisted in completing the benefit request must provide their contact information, sign, and date the benefit request in the section indicated.
If the person who helped interpret or prepare the benefit request is an attorney or accredited representative, he or she must determine if the level of involvement and rules of professional responsibility require him or her to submit a signed and completed Notice of Entry of Appearance as Attorney or Accredited Representative (Form G-28) with the benefit request. If the person intends to represent the benefit requestor before USCIS, he or she must submit a completed Form G-28. The attorney or accredited representative of the benefit requestor cannot serve as an interpreter during the interview.[1]
Footnote
[^ 1] Officers cannot make exceptions for good cause.