I think this bug is new in the 1.9.x series; I didn't see it in 1.8.x. I'm running Affinity Photo 1.9.1 on macOS 10.14.6.
I have a right-hand side panel, and the middle section controls don't go "all the way across". Here's a full screenshot in develop persona for context, along with just the middle panel in the photo persona. It also affects develop persona tones/lens/details, and photo persona adjustment/effects.