How to reproduce:
1. Open the attached project "1242593.zip"
2. Open the "Test" Scene
3. Right-click GameObject "Slider (10)" in Hierarchy window (under "Prefab1" > "Canvas") and select "Added GameObject" > "Apply to Prefab 'Canvas'" in the context menu
4. Repeat step 3 with a remaining "Slider" GameObjects in a random order

Expected result: each applied GameObject is moved to directly below the last GameObject that is a part of a Prefab
Actual result: applied GameObjects are moved to a seemingly random position in a Prefab

Reproducible with: 2018.4.24f1, 2019.4.2f1, 2020.1.0b13, 2020.2.0a15

Note:
- The issue only occurs in an instance of Prefab where GameObject was applied