SUMMARY
When creating a slideshow video file with Tools>"Create video slideshow" and using the "ken burns - camera pan left to right" display effect together with the "Fade" image transition leads to "jumping" images after the fade effect.

The face effect fades into the centered image, but the ken burns effect starts with the image moved to the right. When the fade effect ends, the image jumps from center to the right.

STEPS TO REPRODUCE
1. Select two images
2. Tools > Create video slideshow"
3. Image selection: Images, next
4. Next
5. Video settings:
5.1. Effect displaying images: Ken Burns - camera pan left to right
5.2. Transition between images: Fade
6. Generate and view video 

OBSERVED RESULT
When the fade-in effect is over, the image jumps from the center to the right, because the ken burns effect starts with the image on the right side.

Example video: http://tmp.cweiske.de/digikam-bug-slideshow-transition.mp4

EXPECTED RESULT
Images do not jump around but move and transition smoothly.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 20.04 with Mate desktop 1.24.0-2
KDE Frameworks 5.68.0
Qt 5.12.8 (built against 5.12.8)
The xcb windowing system

ADDITIONAL INFORMATION
ffmpeg info:
ffmpeg version 4.2.4-1ubuntu0.1 Copyright (c) 2000-2020 the FFmpeg developers
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  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100