To clarify first: It is not a question of the duration of the fade. It all works properly in the last part of the clip. Here the fade starts where the slope is shown up. Thats fine. But at the end of the first part, the slope is shown in ver. 17.10 but not in 17.12 (same .mlt file loaded), and there simply is no fade out at all in both cases.
BUT: (and this doesn’t work for ver. 17.10 as the slope does not disappear when the filter is removed (another bug?)) When I clear up the filters pane and only use the throbber for the fade out and afterwards remove the fade out for the video (because it appears for both, video and audio), I come to the result I want. So, thanks for your tip so far! I can use it as a workaround.
But I still believe there is a bug at that point as it should also work the way I described first!