Hi @AlexeiSo ,
I did finally nail down the story on this. OneNote 2010 and later versions (might be in earlier versions as well but MS-ONE only scopes to OneNote 2010) do allow for both jcidOutlineElementNode and jcidOutlineGroup child elements of a jcidTableCellNode. They at least consider the file format valid when this happens. I'm not sure how to accomplish this in OneNote UI. I believe the UI works to remove these in most cases but it is considered technically valid.
Therefore, I will file a bug against MS-ONE to propose the addition of jcidOutlineGroup as a possible jcidTableCellNode child to the specification so as not to confuse implementers parsing and validating.
Let me know if you agree with this.
Best regards,
Tom Jebo
Sr Escalation Engineer
Microsoft Open Specifications Support