When using svg marker symbols and set the path to svg file via Data defined override the svgs display well in the map canvas, but not in the layers panel (applies for QGIS 2.x and 3.x).
svg marker symbol setup:
Expression for Data defined override (Note the output preview):
@svg_path || '\\baustelle_laufend.svg'
(This is build from a variable holding different paths to svg symbols in various environments and the svg file name itself)
The correct display in the Symbol Selector and in the map canvas as well proves that this approach works fine, but in the layers panel the svg symbols are replaced by some ugly question marks:
Why is that so? Is this a bug?