Microsoft Word’s Autocorrect feature has long been hazardous. For example, it will change (c) to ©, turning a 501(c)(3) educational nonprofit corporation into a 501©3 corporation. However, you can go to the Autocorrect options and deselect transformations you don’t want.
One choice is “Automatically use suggestions from the spelling checker.” It’s a good idea to always keep this turned off. Here’s why: not only will it change words it doesn’t recognize into words it does, when a word breaks at the end of a line, it may change the first portion of the word into something else.
I discovered this when, at work, we got another Word “upgrade” and I neglected to uncheck “Automatically use suggestions from the spelling checker.” The unbroken word identification was at the beginning of a line, leaving a gap at the end of the previous line. After identifi, I inserted an optional hyphen (to allow the word to break if it fell at the end of a line and there was room for it to split—exactly the case I faced). The word split, and to my horror, identifi- changed before my eyes into identify-. Word didn’t change cation—it’s a word that means “positive ion,” though I’m not sure Word knew that.
To turn off this menace, go to More Commands, Proofing, Autocorrect Options, and uncheck “Automatically use suggestions from the spelling checker.” If you suspect that Word has already done some damage, turning unbroken words into really broken words, you can use the Find and Replace menu; choose Special, Optional Hyphen. That way you can search a document for words that have split, and you can correct any that Word has already ruined. It appears that if you correct the word manually (I changed identify- back to identifi-), Word will leave it alone.