QGIS » QGIS Application

Text copied from question on GIS Stackexchange: https://gis.stackexchange.com/q/207990/81764
The issue was first described as occurring in QGIS 2.14, but the original post was updated to say that it persists in QGIS 3.0

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:
Units: millimeter

(image showing that arrow symbol previews displayed in layers panel are far too long - apparently at least 6 inches)

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

(image showing that the issue persists when units are switched from mm to map units)

UPDATE
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:

(image showing that arrow symbols are rendered as only the triangular arrow heads in the properties/symbology window)

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.

- Original post on GIS Stackexchange by Jochen Schwarze (https://gis.stackexchange.com/users/63747/jochen-schwarze)