To reproduce:

1. Download attached project "BugTestSortingGroup.zip" and open in Unity
2. Open "BugTest" scene
3. Enter Play mode
Note: Observe that no Sprites are rendered on blue cube in Game window
4. Press "Fix Sort" button
Note: Observe that now Sprites are rendered

To reproduce from a scratch:

1. Create a Sprite Mask
2. Set the Sprite Mask to Custom Range from sorting order 10 to 20
3. Add as a child a SortingGroup and put the sorting order to 15
4. Add a Sprite with sorting order of 0 as a child of the SortingGroup and set mask interaction to visible inside.
Note: At this moment the Sprite is correctly inside the mask.
5. Duplicate the Sprite and move it sightly
Note: Notice that the new sprite does not appear inside the mask.

Notes:
- This issue appears on Windows and OSX
- Workaround: Sprites are being created but they do not appear because they are not taking into account the sorting group. The sorting group needs to be deactivated and activate to workaround this behavior

Reproduced on Unity 2017.1.1.p2, 2017.2.0f1 and 2017.3.0b1