This isn't a bug in the settings editor itself, but for the two children of xfce4-keyboard-shortcuts (xfwm4 and commands), the property and value fields are transposed. That is, the keybinding is the property and the action is the value, making it rather hard to change keybindings here! For xfwm4, these should be in the other order. The actions that xfwm can execute are immutable, but could be assigned to different keys. For commands, it is less clear. One may want to edit both the keybinding and the command associated. The current interface only lets the value field change.
This is not a bug, it's a technical decision made to implement the shortcuts stuff. You should not be using xfce4-settings-editor to edit your shortcuts in a convenient way anyway. Thank you for your bug report, please report any other issue you may find.