Bug 409764

SUMMARY
I assemble a long video from a number of clips. All clips play okay, with VLC as well as in the clip monitor. In the rendered result, however, the first clip is totally degraded throughout. 
I have re-done the rendering, thinking of a glitch: same result
I have imported the story into the most recent appIm, 19.07.70, exactly the same result. 

I can't upload all files (close to 20 GB). How can I help further with this annoying thing? Annoying to me, because it only effects a single, in this case first, clip. Casual scrolling through the rendered result did not bring this defect to light in the first control scan of the ready product. 

STEPS TO REPRODUCE
1. Import a specific set of clips, all playing perfectly well
2. Assemble those clips on the timeline
3. Render the result

OBSERVED RESULT
Rendered video is totally distorted throughout the scenes coming from the first clip.

EXPECTED RESULT

Proper rendering (what else?)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Kubuntu 18.10
(available in About System)
KDE Plasma Version: 5.13.5
KDE Frameworks Version: 5.50.0
Qt Version: 5.11.1