It is in the check list:
https://www.uscis.gov/sites/default/files/document/forms/attachments.pdf
If you have taken any trip outside the United States that lasted 6 months or more since becoming a Lawful Permanent Resident, send evidence that you (and your family) continued to live, work and/or keep ties to the United States, such as:
An IRS tax return "transcript" or an IRS-certified tax return listing tax information for the last 5 years (or for the last 3 years if you are applying on the basis of marriage to a U.S. citizen).
Now granted it says to do this only of there were trips exceeding 6 months. However, when I filed my own N-400, DIY, I lurked here for months, and the advice was to include tax transcripts.
There are enough other conditions listed in the check list that it is not surprising some applicants can blind sided by some ISOs.