This bug has been around ever since the Terrain Editor was switched from a plugin to a proper window. If it’s not a bug, it’s still a design oversight and it consistently proves to be a hitch in my flow.
The key combinations in question are Shift+Mousewheel and Ctrl+Mousewheel, to change the size and intensity of your brush respectively.
Selecting any tool from the Terrain Editor window, moving your mouse into the 3D Scene and then attempting to use either key combination will result in the Studio camera just zooming in and out like normal scroll function.
Left Clicking the world to re-focus the 3D scene will enable you to use the terrain brush hotkeys, however, it will also perform the action of whatever tool you’re holding.
The only way to re-focus the 3D scene without using your brush is to Right Click the world. This is unintuitive. Studio should know that I have a terrain brush selected and allow the terrain brush modifier hotkeys to override camera scrolling.