Hi Lauren. I am an Excel user like you.
My Find/Replace action does not change any formatting in the cell where the contents are replaced by default, but I was able to duplicate the result so here is something to check.
In the Replace dialogue box click on "Options>>". Following the "Find what" and "Replace with" is now a formatting option that should say, as below, "No Format Set". If there is anything else there, click on the dropdown arrow, indicated below, and select "Clear Replace Format". It should then read "No Format Set" and the formatting should not change when you replace data.
Reply if you have additional questions or information. Please mark this reply as answered if this solves your question.
Rich~M