There may be a mismatch between your App.Height, App.DesignHeight and the device.
Currently I am using more advanced layout techniques, because the default was too unpredictable, as it scales depending on client device DPI setting.
On Windows for example, if 2 machines with 15-inch, full HD screens have "Scale and layout" set one to 100% and the other to 125%, the app will look wildly different. You may be experiencing the same.
You can try just setting the widgets width to App.Width and fractions thereof using formulas rather than literal values.
If it is still not enough, you may have change your app settings.
Please note that these are advanced positioning techniques that will give you way more freedom and will your app layout to flow, , but also require more dedication to set positioning with formulas.
I leave my apps at the default 16:9 rather than setting a fixed size (DesignHeight, DesignWidth).
I also disable Scale to fit, Lock Orientation. I disable Lock rotation If I want my design to flow from horizontal to vertical on mobile devices.
On the first screen, I also change the height and width from Max(App.Height, App.DesignHeight) and Max(App.Width, App.DesignWidth) to respectively App.Height, App.Width.
This will allow your canvas to occupy the available space even if it is not full screen, and the scrollbars should show on the widgets.
You can then use formulas like Width: App.Width * 0.5 to make a widget occupy half the screen width and have if flow with the available space, be it a browser window, or the available width of the device running the app, or when you embed the app in e.g. a Teams tab or Sharepoint.