How to reproduce:
1. Open the attached "1239517 repro" project
2. Open the "TestScene" from the Assets folder
3. Select the "Image" GameObject
4. Enter Play mode

Expected result: the Rect Transform values of the "Image" GameObject don't change
Actual result: the Rect Transform values of the "Image" GameObject change steadily

Reproducible with: 2017.4.40f1, 2018.4.22f1, 2019.3.11f1, 2020.1.0b8, 2020.2.0a8

Notes:
-The GameObject holding the Scroll Rect needs to have a parent GameObject between itself and the GameObject holding the Canvas Component
-This bug doesn't reproduce if the Canvas is enabled when entering Play mode and gets disabled while playing
-The speed at which values change is directly correlated to the Rect Transform Y Scale of the GameObject holding the Scroll Rect Component. The closer it is to 0 the faster the values change. If the value of the Y Scale is 0, the bug doesn't reproduce
-When the Canvas gets enabled the values rapidly return to the correct ones as shown in "ScrollBug.mp4"
-If the values become big enough they won't return to the correct ones even when the Canvas is enabled as shown in "ProlongedTest.mp4"
-Sometimes the values change even when not in play mode as shown in "NotInPlayMode.mp4" but this happened only a few times and the reproduction steps for that remain unknown
-The Canvas Scaler seems to be a workaround and not a cause of this bug since it is reproducible without it