It is truncating; actually, your UI is not responding to the resolution of your device. You have to set its anchoring and scaling. I would recommend you read the Unity guide for "Designing UI for Multiple Resolutions"
For a reference, this what you have to change:
In the Canvas Scaler component, you can set its UI Scale Mode to Scale With Screen Size. With this scale mode you can specify a resolution to use as reference.
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When the screen size is changed to a larger or smaller resolution, the buttons will also remain anchored to their respective corners. However, since they keep their original size as specified in pixels, they may take up a larger or smaller proportion of the screen. This may or may not be desirable, depending on how you would like your layout to behave on screens of different resolutions.