Steps to reproduce:

1. Open attached project "778125.zip"
2. Open scene "repro"
3. In hierarchy, select GameObject
4. Notice that it has two components: PolygonCollider2D and Test script with public field that accepts Sprite
5. Drag "sprite" from project tab to the Sprite field of the Test script
6. Notice that PolygonCollider2D is reshaped

Note: this doesn't happen if:
1. In inspector, Test component is higher than PolygonCollider2D component
2. Sprite isn't added by drag and drop but rather by using "Select Object" window

Workarounds:
1. Make sure that Test script is higher than PolygonCollider2D
2. Add sprite not by dragging it in but by using "Select Object" window

Reproduced with: 5.1.4f1, 5.2.4f1, 5.3.3p3, 5.4.0b9