To reproduce:
1. Create new Unity project
2. Create any public field on any object (e.g. UI > Canvas, set Render Mode to Screen Space - Camera)
3. Drag and drop any object to the public field

Expected result: the field is highlighted when dragged on it
Actual result: the field is not highlighted

Reproduced on:
2018.2.0b1

Not reproduced on:
2018.2.08

Notes:
This behaviour happens on any public field. Also tested with a simple script with public variable, same issue.