So here is the deal. You are filling out fields using Safari. It could be your name, email address, street name, or whatever. That word, or part of that word, is similar to an existing word. Safari changes your word. You change it back. Safari changes your word. You change it back. etc.
This happened to me all the time. One of my email addresses starts out with “ngravley”. Safari changed it to gravely.
If this is a problem for you here is a simple way to solve it. Open Pages and type in the word or combination just the way you want it. It will probably be underlined in red. Then select Edit > Spelling > Spelling. Your word will show up in a window. You may or may not get alternate spelling suggestions. Just select Learn from the right hand menu and the highlighted word will be saved and will become effective for all the Apple related apps wherein spelling word suggestions are automatically inserted on your computer.
Oh, and if anyone should care, Fenris is the name of my brand new grandson.