You're right, thanks. When creating a new transition, its duration is fixed at 2 seconds, and only a portion of the video is displayed (for your video, that's 2 seconds out of 10 seconds).
After the user applies a custom transition, all objects are automatically scaled (including the video speed) to match the user-selected duration. This will also ensure the display of animations and video objects is not disrupted.
We'll try to add a new parameter to the custom transition editing window: transition duration.