Relationship lines

You’ll find it’s caused by the relative position of the two bars in the "Bars" setup screen. The relationship lines will default to the higher listed bar.

Right click on the gantt area and take the "Bars" option. You will now see all the available bars with your choices checked. Try to visualise this as a stack with those nearer the top covering/overlaying those below when finally displayed. Your baseline bar is most likely sitting above the "work" bars at present. Highlight it and use the "shift up" or "shift down" buttons on the r/h side to demote it to a suitable point.

Richard,
thanks a lot, it was difficult to imagine this was the reason
Alexandre