That's not a bug that's a theme gtk3 themes missing codes inside gtk.css. If you remove gtk-3.0 files & folder off and remain gtk-2.0 folder inside theme will display properly but that means no gtk3 support you've to use gtkrc from gtk2.0 to control that theme.
You don't see on lxterminal and other because they're not using gtk3. You can check by install Uget and you'll see it broke as xfce4-terminal. It affect on gtk3 apps that didn't support gtk2 & gtk3 properly.
I've been played around with Fedora, Xubuntu, Debian & other Xfce distro and saw the same. There's nothing you can do than change to the other theme that support gtk3 theme for Xfce. Like Numix, Blackbird, Greybird these worked. But Bluebird and other gtk3 themes ain't worked with Xfce.