Hi, as said on irc, when the tasklist is configured to group tasks, the whole group should flash in response to the urgency hint of one application in the group.
A reproducer from the duplicate bugreport: "To reproduce, configure the 'Window Buttons' plugin to always group windows. Now start a single instance of xeyes, minimize it and run the following command: 'wmctrl -r "xeyes" -b add,demands_attention'. Note that the minimized icon flashes. Close this instance of xeyes. Now start two fresh instances of xeyes, minimize them and run the same wmctrl command, note that the grouped icon does not flash."
*** Bug 8165 has been marked as a duplicate of this bug. ***
Simon Steinbeiss referenced this bugreport in commit 632d5f1bc7cd564be3418bf7549e2f749e4f7db1 tasklist: Fix urgency blinking for group buttons (Bug #6904) https://git.xfce.org/xfce/xfce4-panel/commit?id=632d5f1bc7cd564be3418bf7549e2f749e4f7db1
Sorry it took so long - especially considering the fix was not that hard...