How to reproduce:
1. Open the attached "ScrollRectBug" project
2. Open the "Game" scene and play it, notice that the Scroll Rect is clamping Canvas movement in the content correctly
3. Open the "Repro" scene (which loads the "Game" scene) and play it, Scroll Rect is clamping Canvas outside of the content

Reproducible: 5.4.1f1, 5.5.0p4, 5.6.0b3
Not Reproducible: 5.3.7p3

Note: The issue persists in the build of the project, even if you remove "Repro" scene from the Scenes In Build and leave only the "Game" scene

The issue seems to be caused by the scrollview's rect value during ScrollRect.UpdateBounds(), which has the right value when the scene is loaded directly, but is (0,0,0,0) when loaded additively