This was already reported as part of https://bugzilla.xfce.org/show_bug.cgi?id=13962. If you add a launcher that has an icon file instead of a name in its desktop file, the pixbuf will be created correctly, but upon resizing the panel, the launcher's icon will not resize anymore. This is due to the switch from xfce_panel_image to GtkImage, which provides resizing for icons basd on icon names, but not for pixbufs. So either we switch back to xfce_panel_image which had all the necessary resize magic for pixbufs (and maybe try to fix its sizing to be on par with the new xfce_panel_plugin_get_icon_size call) or we have to re-implement all that magic on top of gdkpixbuf by attaching to the size_changed callback.
Simon Steinbeiss referenced this bugreport in commit f8441b8bfa4c945293f4def5cfd9a3591f79d8d4 launcher: Fix resizing of pixbuf based icons (Bug #14138) https://git.xfce.org/xfce/xfce4-panel/commit?id=f8441b8bfa4c945293f4def5cfd9a3591f79d8d4