Yes, Last Friday .I had the same issue, but with IMM 5532 . I forgot to attach additional sheet for the sponsor's employment history .I believe they do add the details before completeness check if they receive the webform on time .Since I have submitted the file on September 26th ,I believe they would get it as AOR usually follows 2 months after submission.
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Raised a webform the next day ,but yet to receive acknowledgment .For UCI ,I just put 0000000, and rest all details of the applicant as per the application submitted.
I used the reason box for explaining the mistake ,and attached the files .I was confused in the last step-in which, it asks for confirming birth date and email address. Since I am the sponsor ,I put my details in it .