Yep, tested it for myself even tried making the rounding to the max. A work around would be to use a rounded background of your desire then make then scrolling frame transparent and add all your goodies to the scrolling frame.
They’re not poorly optimized, but it has to do eith how UI is rendered. Scrolling frames work best with ClipsDescendants enabled, and if you were to add a Frame with a UICorner and that property enabled, you’d see any child frames will not clip. Same goes for rotation on GuiObjects. Unsure about gradient though.
This isn’t nothing new just how it was made. If you want UICorner on scrollingframes just parent the scrollframe to a regular frame and put the UICorner in the top frame.
According to the Roblox Documentation, as @GetStyled said, it mentions that UICorner can not be applied to a ScrollingFrame, therefore this is not a bug, it’s something that Roblox just doesn’t support at the moment.
If you would like to make a ScrollingFrame have UICorners, you can either use a CanvasGroup, then parent the ScrollingFrame there, or use a frame and parent the ScrollingFrame at the frame, as Garlic said.
There is a work around. It’s as simple as having the desired ScrollingFrame (Scrolling frame being transparent (or however you want it)) over a Frame with the UICorner that you want
