Not sure if this should be classified as a bug but I recently explored the Sleek UI and I do like the cleaner arragement.
However, a deal-breaker for me was that I can’t continuously add attributes in classes in class diagrams. That is, I have a class and add an attribute. In the classic UI, once I hit Enter, the UI creates the next attribute so that I can continue to enter attributes without breaking focus. In Sleek UI, focus of that class is lost immediately and so I have to re-focus the class and add the next attribute manually again.