There seems to be a problem with generation of thumbnails for the first frame of some clips on the timeline or the storyboard.
Here is a simple formula to demonstrate to issue:
If you have the physical version, you will have the _Demo project in Import/My media/Projects/
Open a new movie and switch to Storyboard view
Locate the first three MXV files in the _Demo project and send or drag them to the movie area.
If the Movie settings message comes up it doesn't matter if you selecct adjust or ignore.
The third clip clip_003 has the wrong thumbnail - it has the same thumbnail as clip_002.