In QGIS 2.14.5 LTR (and recently in QGIS 3.0) I've set up a beautiful map design with a graduated renderer and vector field markers for each value range (arrow length is directly proportional to stream velocity, which ranges from 0.001 -~5m/s).
This worked great, but I wonder if I missed a setting to shorten the arrows in the legend. With a 'normal' width layer window, the class values cannot be seen:
My first guess was, that it has sth. to do with switching arrow length, line width units from millimeter to map units, but this doesn't affect the legend:
Units: map units
Issue still persists in QGIS 3.0.
I created a screencast to visualize this: https://youtu.be/oxJ3tGXFB18
In properties / symbology the symbols render correctly:
Note that the layer list displays the markers correctly, when duplicating the layer or applying changes to the style as long as layers panel width does not change.