This is a minor UI bug that does not affect any functionality.
When I was hovering over the title bar the icons of the close, minimize and zoom button only appeared when I was slightly above the actual buttons. This may be an issue because the title bar changed its height from Catalina to Big Sur.
Bug occurred on: macOS version 11.0.1 (20B29) and Affinity Designer 1.8.6 (and Affinity Photo 1.8.6 + Affinity Publisher 1.8.6)
afdesign_bug.mov