Actually this might be an Obsidian bug on MacOS. Hidden scrollbars is a MacOS feature.
But when you set it to “Show scroll bars: Always”, Obsidian still auto-hides the scroll-bar. (Sandbox vault, MacOS 12.3, Obsidian 0.16.2 and 0.16.3)
Edit: When turning on “always show” on scrollbars in MacOS, you have to restart Obsidian.