To reproduce:
1. Open attached project;
2. Open "Scene" scene;
- Console logs: "Deserialized as 0" and "set to X" (a random number);
3. Reimport "MyScript".

Expected result: after reimporting a Script, private variable "MyProperty" should be set to 0.
Actual result: after reimporting a Script, private variable "MyProperty" is set to a random value it just picked.

Notes:
-This only happens in edit mode and only while the editor is open. Restarting the editor causes "MyProperty" to be set to 0 again.
-With a string property, this also means that Unity initialises it as "" where you would expect a non-serialized property to be null.

Reproducible on versions: 5.3.7p3, 5.4.3f1, 5.5.1p4, 5.6.0b8.