@Esteban Hugo Somma ,
Welcome to Q&A forum!
Based on your description, I tested and reproduced your issue on OneNote 2016/Win 10.
I fully understand the trouble this issue brings to you, and as far as I know, the Merge Cells is not available in OneNote currently.
I would suggest you go to File>Feedback in OneNote or go to this page to submit this issue to Microsoft directly.
Thanks for your understanding!
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