I have Word via Office 365. I have unsuccessfully tried to create a template document with the styles I need for my reports. Once created (from scratch) the font style and size changes automatically and even lines of text disappear. I have started from scratch again, watching a YouTube guide (following the steps as the video does it) and see that my Word documents do not follow those of the person in the video. In particular the numbering doesn't automatically update.
I may have disrupted the original styles (in my ignorance) and so maybe this is undermining my own efforts... just a thought.
Usually, paragraph styles wouldn't change like that. However, there are some properties of styles that can cause (seemingly) unexplained changes. For example, if a style is based on another style and the "parent" style changes, the "child" may change as well. Another possibility is that the style is set to update automatically, which means that its font (for example) changes as you apply a different font to text formatted in that style.
It is difficult to tell what has happened with your specific document. If you want to, you can share a sample document.