To reproduce:
1. Open attached Unity project "slashGUI.zip"
2. Example --> Editor GUILayout Popup Usage
3. Try to select another option from the dropdown menu

Expected result: "/u002FSphere" and "/u002FPlane" should appear in the dropdown menu
Actual result: There are no options to select from the dropdown menu

Reproduced in: 5.4.5p1, 5.5.3p4, 5.6.1p1, 2017.1.0b7, 2017.2.0a1

Note: The options are written in "gui" script line 8
Note: Everything works perfectly with slashes removed
Note: Removing slashes from "u002FPlane" and "u002FSphere" but leaving a slash in "u002FCube" results everything appearing to be normal, but once you choose
either one of the options, you can no longer select "/u002FCube"