Created attachment 5544 panel screenshot xfce4-brightness-plugin crops the xfpm-brightness-lcd icon when the panel size is inferior to 28px. It is not noticeable with the default icon, which is small enough.
The brightness plugin has been dropped and the (remaining) battery plugin has received a patch to fix this issue. http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=8533d1496ad04a121eecf5a1e874564d2e56cfd7