It's been many years since I used the Rating field, and the Stars that show the Rating value, because I kept losing the Rating I had set. I realized that the Stars indicate the value in the Rating field, AND they can change that value by touching a different star. But there's a risk, because the Stars appear in various places (tag window, any view or list that has that field, etc), AND they will change simply by touching. This is unlike other fields that are changed by first selecting the field/tag, then typing or selecting from a list.
Is it possible that the Stars are being touched inadvertently, such as to scroll/drag a window or list, or when accessing an adjacent field/tag, thereby changing the Rating value? There may be other scenarios that will alter the Rating field value.
That problem happened to me so many times that I gave up on the Rating/Stars field and added my own custom field to do the same, calling it Grade (as in A, B, C, etc). I've never had a Grade value get changed accidentally. (Another benefit turned out to be using the field for odd Grade values that would not correspond with the numeric values of Rating/Stars.)
Or, if you find a different cause or solution, please share it.