As of the last commit to create the new icons (org.xfce.panel.*), the 48x48 icon doesn't have a transparent background but rather a white one. On dark themes the white background is visible.
Simon Steinbeiss referenced this bugreport in commit 9840226519fa71b739358e60a955e8fa70b1625c Fix background of 48px app icons (Bug #16873) https://gitlab.xfce.org/xfce/xfce4-panel/commit/9840226519fa71b739358e60a955e8fa70b1625c
Thanks for noticing, adjusted the svg and the png export.