Grouped windows reorder themselves at the start of Window Buttons list

I have a panel at the top with the Window Buttons plugin added with these options:

• Sorting order: None, allow drag-and-drop
• Window grouping: Always

Let's say i ordered my windows like this: [Firefox][Thunar], then i open a new Thunar window.

What happens:

• The buttons rearrange themselves like this [Thunar(2)][Firefox]

What should happen:

• Buttons should keep their order like this [Firefox][Thunar(2)]

Closing the newly created Thunar window restores the buttons order to [Firefox][Thunar]. Here's a short video showing what happens:

movingButtons

$ xfce4-panel -V
xfce4-panel 4.14.3 (Xfce 4.14)

$ lsb_release -ds
Ubuntu 20.04.1 LTS