Steps to reproduce:

1. Open attached project "789063.zip"
2. Open scene "repro"
3. Drag sprite "Circle" to the hierarchy
4. In game view, notice that sprite is visible as expected
5. Press play
6. Notice that sprite disappears. Inspect sprite and notice that its sprite renderer no longer has a material
7. Now set Y rotation to 180 and notice that sprite became visible and now it's pink, which again indicates that it doesn't have material and was flipped

Note:
-Flip happens only with Polygon sprites

Reproduced with: 5.4.0b14, 5.4.0b15
Not reproducile: 5.3.4p3, 5.4.0b13
Regression introduced in: 5.4.0b14