This problem is in some way similar to the one I already solved before https://forum.freecadweb.org/viewtopic.php?f=17&t=50862. Under certain conditions constraints can't be selected from the merged constraint icons. It happens independent of zoom when sketch is not in XY plane. The reason is that icon position is given in sketcher coordinates but method getCoordsOnSketchPlane is used to project point given in global coordinates to the sketch plane. So it makes no sense to use sketcher coordinates with it. To fix it I manually map sketcher coordinates to global by applying the same rotation and translation as the sketch. Global coordinates are required in order to project point to the screen.
git commit 4516547c8743e44b08441a924f9220efb8b4cd14