Steps to repro:
1. Open the attached project and 'Boot' scene on Mac machine with Metal API;
2. Enter Play mode;
3. In the Game view press "Load Setup 1" button;
4. In the Game View press "Unload Setup" button;
5. In the Game view press "Load Setup 1" button;
6. Repeat steps 3-5 10-12 times.

Actual result:
After several loading/unloading scene various artifacts are visible in the scene. See attached video.
Regression introduced in Version 2019.3.0.

Expected result:
The additively loaded scene is rendered smoothly without artifacts.

Notes:
- Reproducible on macOS only; Not reproducible on Windows;
- Reproducible in Version 2020.2.0a19, 2020.1.0f1, 2019.4.6f1, 2019.3.16f1;
- Not reproducible in Version 2019.2.21f1;
- Could not repro in 2019.1 and 2018.4 as the UI is broken.
- Reproducible only in Metal API; could not repro in Open GLCore;
- The issue might not reproduce every time; in case it doesn't repro please try loading unloading scenes more times and varying the time between loading and unloading scenes (by making it faster or slower).