Yep, did it again and the answer is: Maman mange un œuf.
Not cool, because my english keyboard can't make those melted words and I don't think there is a difference in the words.
Or is there between oeuf and œuf?
Only recently has the system started to reject "oeuf" without the ligature. So every sentence with "œuf" needs to be corrected to include "oeuf" as well. And the same story repeats with "sœur/bœuf/œil...".