Reproduction steps:
1. Open the attached project.
2. Build it to iOS 11 device.
3. Click Start Recording.
4. Wait a minute.
5. Click Stop Recording.
6. Click Preview.
7. Click Save.
8. Open Camera Roll.
- Notice, that the video is not saved.

Workaround 1:
9. Repeat steps 3-6.
10. Click on the share button (box with an arrow near the Save button).
11. Click Save Video.
12. Open Camera Roll.
- The video is saved.

Workaround 2:
13. Switch back to the application.
14. Click Save.
15. Open Camera Roll.
- The video is saved.

Reproduced with:
2017.1.0f1, 2017.3.0a4
iPhone 6s iOS 11.0

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Seems to be fixed in iOS 11.2 Beta