For many, many, many years - the Unfold Mapping feature in the Unwrap UVW modifier randomly decides to split every edge in the object. You can literally run Unfold Mapping on the faces of a cylinder 10 times in a row and get different results every single time. There is no rhyme or reason to how buggy this integral - yet broken tool is. It is - and has been - broken for at least 10 years now. For the love of all that is holy - fix Unfold Mapping.
