SUMMARY
Hello,
first, thank you for developing kdenlive. It is great to have such a feature rich open source video editing software. I only have one problem since a long time and always searched for a solution on the internet. But it seems I am the only one encountering it.
I have linux Ubuntu noble and the problem occurs with all kdenlive Versions 23 to 25 I used in the last year. The current version 25.03.70 has the same problem.
Sometimes, without any regularity mixes only apply to Audio and not at all any more to video. If that happens with a project it normally stays like that and then I change to composite transitions which is a lot of work and unintuitive. I now have a bigger project and the problem occurred just after a few added clips. No matter if I press U, the Button or do the double click, the result is the same: The Mix applies to audio but not to the video and there is no message.
One time a project somehow recovered and it suddenly worked again. But all the other times it stays not working.
I hope to get some solution as especially with the big project it is very problematic. I just ignored it for now and hoped that it starts working again after working on the project for a long time and recovering like with the other one. But it still doesn't work.
I am glad for any support!

STEPS TO REPRODUCE
occurs randomly, but when it happens mixes are not working any more in that project forever. Only one exception where they suddenly started working again.

OBSERVED RESULT
Mixes only apply to the audio track, not to the video tracks any more.

EXPECTED RESULT
Mix should apply to the video and audio track.

SOFTWARE/OS VERSIONS
Ubuntu noble
Kdenlive from 23 to 25