First off sorry, something went wrong, can't edit the description now, so it goes here, in a comment. :( If you activate window-grouping in the window-buttons plugin, the focused group never gets the "active/focused" state in the window-buttons, i.e. it's impossible to see in which group the currently focused window is.
Simon Steinbeiss referenced this bugreport in commit f8d673c5024af0cd6b7f2bb63f2ffa434a708faa tasklist: Make grouped buttons active (Bug #9129) https://git.xfce.org/xfce/xfce4-panel/commit?id=f8d673c5024af0cd6b7f2bb63f2ffa434a708faa
Great, I love talking to myself :)