SUMMARY
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Hi ! I have noticed several issues related to the use of "image sequence" clips. The main one being that, if the user forces the generation of a proxy (aka intermediate clip) by right clicking on the clip, the proxy is used instead of the original files in the rendered video EVEN when unticking "use proxy clips" (or use cached files) in the rendering options.

The behaviour is correct with regular video files with proxies. If I mix "image sequence" clips & video clips, only the "image sequence" clips are low resolution in the rendered video.
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STEPS TO REPRODUCE
1. Create a new project. Enable proxies.
2. Add an image sequence clip.
3. As no proxy is automatically generated for this kind of clip, right click on it and force the proxy clip / intermediate clip create
4. Render the final video and make sure "use proxy" (or use cache file) is unticked

OBSERVED RESULT
5. The final video, despite being in the target resolution, clearly shows a 640*480 sequence where the image sequence clip was placed

EXPECTED RESULT
6. The rendered video should have made use of the original pictures files from the image sequence clip and not the cached / pre-rendered / scaled down version

SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: KDE Neon 5.27 / Linux 6.2.0 
(available in About System)
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.108.0
Qt Version: 5.15.10

ADDITIONAL INFORMATION

This issue raises 2 other issues :

- I should not have to use proxy clips for a simple image sequence but the system crawls when previewing it, making it impossible to record a voice over (7th gen i5 CPU)
- proxies do not seem supported for this kind of clip but I can force the generation of proxies. Strange thing : if I right click on a clip to force the proxy generation, when I right click again, it is still "unticked" and no "P" is displayed on the clip -- while it clearly has been generated (in the cache directory).

THANK you so much for your help ! :)