AutoCorrect, AutoFormat as you type, AutoFormat, AutoFormat Math have no effect on what happens when you open a document.
All except AutoFormat only work with text you type. AutoFormat only applies if the command is used and modern versions of Word do not have the command readily available.
Are these being marked as tracked changes?
Are there fields? They will update in some cases upon opening a document and will be marked as a tracked change.
If a document has "Automatically update styles from template" checked in the Templates and Addins dialog styles will change. This should be unchecked as a general rule.
Are these the only changes you see upon opening a document?
You are welcome to attach a document with the problem. (Paperclip icon in full editor)