SUMMARY
The slideshow produced by adding an image sequence seems to show images with the wrong size when transitioning from an image to the next one. Both in the preview and in the rendered video, when a new image appears, it seems to appear scaled to the size of the previous image. After some milliseconds, the new image is shown with its proper size. I'm attaching a couple of videos to show the result.

STEPS TO REPRODUCE
1. Create a new project (HD 1080p 60fps, but I tried other setups).
2. Add an image sequence.
3. Select a sequence of images and ensure the sequence contains alternating images of different sizes. This is the set I used: https://app.box.com/s/df55s9ldew4sj4nyk6t9h7k7u96fjtuw.
4. I can see the issue both in the preview and in the rendered video, but not for all transitions. The issue did not seem to be entirely deterministic.

OBSERVED RESULT
Resulting video shows the selected sequence of images. In some cases, when transitioning from one image to the next, the following image is shown with a size similar to the previous one, resulting in distortion. After some milliseconds, the geometry is fixed.

EXPECTED RESULT
When transitioning from one image to the next, the next image should always be shown with its own geometry, without distortion.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux Manjaro
KDE Plasma Version: 5.20.4
KDE Frameworks Version: 5.77.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
I followed the steps above and this is the result:
* Video without dissolve transition:
https://app.box.com/s/rcgtxwojmjxnleava475f7eehkogjagy
* Video without any animation: https://app.box.com/s/uaqlftg0sb6xwhc7meiqdu8boi0l123z