How to reproduce:
1. Open the attached "Dropdown Bug Reproduction" project
2. Expand the "Canvas" GameObject in Hierarchy and inspect the "Unrelated Gameobject"
3. In Inspector, disable its "Start Dropdown Broken" setting off and enter playmode
4. Notice that the Dropdown is working correctly, exit the playmode
5. Enable "Start Dropdown Broken" setting on and enter playmode

Expected result: Dropdown list still shows up
Actual result: Dropdown isn't working anymore, list doesn't show up

Notes:
1. Could reproduce the issue only when adding components trough script. Reproducible only if the child GameObject has a parent set and ONLY THEN the UI component is added. Creating a GameObject, adding the UI component to it and then setting a parent GameObject of it has the Dropdown functioning correctly.
2. Can't reproduce the issue when adding Button or Input Field components

Regression introduced in - 5.6.0f1
Reproducible with - 2017.1.0b1, 5.6.0f3, 5.6.0f1
Not reproducible with - 5.6.0b11, 5.5.3f1