Right, as I said, if I change to a fixed start date it disappears. Still, that behaviour didn't occur before and previous plans were scheduled the same way.

I also don't get, why this only happens for that specific task. It doesn't occur anywhere else in the chart.

One of MS Project's big miracles I'll have to live with?
(I can live with just changing it to 100% when done, but still I like working with percentages)