If you choose “whole word” as the find option and do a “replace all”, it will replace partial words.
Example, I had a document with a character named Grenn (two Ns). In some places I accidentally spelled his name wrong, with one N (“Gren”). I wanted to replace “Gren” with “Grenn”, but only whole words. I put “Gren” in the find, “Grenn” in the replace, chose “Whole word” in the “Find Options”, and selected “Replace All”. It did find all of my “Grens” and turned them to “Grenn”, but it also turned my “Grenns” to “Grennn”.
This seems to only be a problem with “replace all”. If my document only has “Grenn” (two Ns) and I do a search for “Gren” (one N) with “whole word”, it does not find anything when using Next and Previous, which is correct. But the “replace all” will still change the “Grenns” to “Grennn”.