Startup movies are skipped when World Composition is enabled. Even with the 'Wait for Movies to Complete' option enabled, the startup movie plays for a very short time, and then the level is loaded and cuts the movie short.
This does not occur when 'World Composition' is disabled.
This behavior is the same in the binary 4.11.2 and 4.12.3 engine versions.
I have provided a video in the comments which illustrates the repro steps and issue occurring.
1. Create new blank blueprint project from launcher.
2. Navigate Project Settings > Movies
3. Check the 'Wait for Movies to Complete'
4. Add a new movie using the plus sign.
5. Create new level and make sure it is loaded in the 'Levels' window.
6. Open the World Settings tab and check 'Enable World Composition'
7. Save All
8. Play in Standalone mode.
Outcome The startup movie added, plays for a short second and then skips and loads the default startup map.
Expected The startup movie completes playing and then the level is loaded.
Head over to the existing Questions & Answers thread and let us know what's up.