prevent resizing with option key
Since this is not standard macOS functionality, neither default nor optional, this is either functionality added by you (or someone acting as administrator) or a third-party app.
You can check what third-party apps have system-wide access to window manipulation in System Preferences → Security & Privacy → Privacy → Accessibility. One by one, try disabling suspect apps and attempting the shortcut again.
When you've narrowed down the specific app, you can either disable the app or research the possibility of changing/disabling the offending shortcut.