While Copy paste the content "d’un", after save and re-open the document, word automatically replaced like "dext" i.e. d[some_other_text].
This can't be identified even in open Xml content or track changes and all are proper unicode while cross-checking. It happens in word 2010. I cross checked the same with word 2016 but it's fine and no changes happens. Please suggest me what is the actual issue.

It happen due to the auto correction of the word, when you save and re-open the document during document loading it will auto-correct it may be!

AutoCorrect is not triggered by saving and reopening, but by typing text. However, this does sound like an AutoCorrect action.

• AutoFormat and AutoCorrect (Tame AutoFormat) by Suzanne Barnhill, MVP
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• Automated Boilerplate Using Microsoft Word

Do you have the AutoCorrect option set to automatically use suggestions from the spelling checker?