I just started using v21.12.0 (Win10) today. For testing I use two photos.
1) Add a Composite transition between two clips.
2) Add a key frame at the end of the transition.
3) Set the opacity to zero.
4) Slide the cursor between two key frames.
The opacity value only jumps to zero on the last key frame.
Expected: The value should change slowly between the key frames.

Is this a bug or I have to check a (new feature) box somewhere?

Just tried to replicate this and can confirm that yes the Composite composition is not working correctly. Anything you set for method does not work at all. Except for position or size values. Even when setting a keyframe at the end of the transition.

However, once you create a keyframe anywhere between the beginning and the end of the composition does the "evaluation" of the Composite Method and any Opacity value start and work.

Thanks for the information. Meanwhile I'll go back to my old version.

berndmj wrote:However, once you create a keyframe anywhere between the beginning and the end of the composition does the "evaluation" of the Composite Method and any Opacity value start and work.

Thanks for the tip.

I had to realized that v21.04.2 only works perfectly in preview. But the effect won't be rendered. v21.12 with a little bug but renders correctly

Please open an Issue about this (if not present) https://invent.kde.org/multimedia/kdenlive/-/issues