Don't use a transition to fade out. Click the FX button at the bottom-left of the clip and choose the TRANSPARENCY (opacity) effect.
Open the preset dropdown and choose FADE OUT. Click the icon with the squiggly line, to expose the animation line.
In the graph, near the end of the clip, drag the blue line upwards. A keyframe will be created when the mouse is released.
This line is the fade trajectory. Drag the keyframe dot left or right to change the fade length. Drag it up or down to change transparency .
To UNDO press <ctrl-Z>. To delete keyframes, place the pointer on the keyframe until it lights up, then right-click and DELETE.