Yes, that happened to me except the I-751 Interview was scheduled for the day after the N-400 Interview. So I called USCIS and they also could not see the letter in the system so I brought it with me to my N-400 Interview, the IO doing my interview thought it was really weird considering I already had an interview for my I-751 more than a year prior. So he took the letter and shredded it. It was an error.