How to reproduce:
1. Open the attached project "825614Canvas.zip"
2. Open and play test scene
3. Stop the scene and check the console
4. Firstly you see the edited plane distance in Start and canvas RectTransform values
5. Later you can see the edited plane distance in Update and canvas RectTransform values
Notice that in Start changing plane distance doesn't affect canvas RectTransform values.
However in Update the transform does indeed change to reflect the new position but only after one update cycle.

Reproducible: 5.2.5f1,, 5.3.6p4 , 5.4.0p4, 5.5.0b2

By Design: Updating positions are done at the end of the frame to cache any and all changes.