My query is really whether I should report this as a bug or whether it's expected behaviour:

I applied a Vignette effect to all the clips in my bin. All appeared fine whilst editing, I was able to see the Vignette in both Clip monitor and Project monitor. On preview rendering the project a number of the clips suddenly appeared black, but not all of them. The vignette was applied to all clips.

I tried rendering out the whole project first to mp4, then to mpeg then to webm, all displayed black holes at various points.

Each time I saved and re-opened the project more clips would become black, audio was still present, but no video showed, just black in both clip monitor and project monitor. After some investigation I decided to start disabling effects. I had some applied to the bin clips and some applied to clips in the timeline. Eventually I discovered that disabling the Vignette effect on the bin clips made the clips appear again.

As I still needed the Vignette I tried applying it to the tracks themselves, rather than individual clips. This worked and I was eventually able to successfully preview render and render the entire project with all effects visible.

So is it expected behaviour for the Vignette effect to not work when applied to bin clips? I assume not because (a) why not? and (b) it did work initially.

Ubuntu 20.04.1
Kdenlive 20.08.2
MLT version 6.22.1

update on this just in case anyone actually cares. Have recently started a new project, I now can't use Vignette or Contrast on clips in the timeline, or on tracks in the timeline, but they both work if applied to bin clips. So basically the opposite problem I had with my previous project.

further update: re-installed kdenlive, deleted config file (~/.config/kdenliverc) - have tried the "Vignette Effect" under Alpha/Transform and it seems (so far) to be working.

Further update as I can't believe I'm the only person encountering this problem!

"Vignette Effect" under the Alpha/Transform menu no longer works for me under any circumstances, whether applied to bin clips, tracks or individual clips. It appears fine in the project monitor but renders out as grey. Nothing on screen, audio present.

"Vignette" under Image/Adjustment does work.

I was also using "Contrast" with a preset adjustment. Using the preset resulted in a render with grey holes. Turning off the preset and applying the same settings manually worked fine.

Again I'll ask - is there somewhere else I should be posting these queries? This is a fresh install with a new config file and this is obviously not expected behaviour so should I be filing a bug report? I don't really understand how support for Kdenlive works, or if it even exists at this point.