As of recently, when a UICorner is applied to a ScrollingFrame, the 3 images that make up the scrollbar of said frame will also have their corners rounded off, when it should just be the frame itself:
Hey so what i recommend is that you put your scrolling frame inside a parent frame
and then add uicorner to the frame, then add uistroke, make it really thick and match the colors
In my experience, hiding the scrollbar when using UICorner usually resolves the visual inconsistencies. While this workaround helps for now, it is not ideal, and having a proper fix from Roblox would make UI design much more predictable and reliable. I hope this issue is addressed in a future update so that developers can apply UICorner freely without needing temporary solutions.