Taskmanager icon grouping not working despite identically WM_CLASS

[Basti]

Im using a program named Xournal++ for which the icon grouping in the plasma task manager isn't working correctly. When Xournal++ is pinned to the KDE taskbar and you try to start it from there KDE does not show the new windows associated with the original icon but instead creates a new one in the taskbar. Further windows are grouped under the new icon.

The WM_CLASS of the window corresponds to the WM_CLASS specified in the .desktop file of the application. I would expect that all windows are grouped under the same icon.

Output of xprop:

_NET_WM_ICON_GEOMETRY(CARDINAL) = 3043, 1028, 60, 52
_NET_WM_ALLOWED_ACTIONS(ATOM) = _NET_WM_ACTION_MOVE, _NET_WM_ACTION_RESIZE, _NET_WM_ACTION_MINIMIZE, _NET_WM_ACTION_SHADE, _NET_WM_ACTION_MAXIMIZE_VERT, _NET_WM_ACTION_MAXIMIZE_HORZ, _NET_WM_ACTION_FULLSCREEN, _NET_WM_ACTION_CHANGE_DESKTOP, _NET_WM_ACTION_CLOSE
_KDE_NET_WM_FRAME_STRUT(CARDINAL) = 0, 0, 36, 0
_NET_FRAME_EXTENTS(CARDINAL) = 0, 0, 36, 0
_NET_WM_DESKTOP(CARDINAL) = 0
_KDE_NET_WM_ACTIVITIES(STRING) = "0965d6be-67ce-4858-a280-fbc6983d5830"
WM_STATE(WM_STATE):
                window state: Normal
                icon window: 0x0
_NET_WM_STATE(ATOM) = _NET_WM_STATE_MAXIMIZED_VERT, _NET_WM_STATE_MAXIMIZED_HORZ
WM_HINTS(WM_HINTS):
                Client accepts input or input focus: True
                Initial state is Normal State.
                bitmap id # to use for icon: 0xb200014
                bitmap id # of mask for icon: 0xb20001b
                window id # of group leader: 0xb200001
_GTK_THEME_VARIANT(UTF8_STRING) = "dark"
_KDE_NET_WM_USER_CREATION_TIME(CARDINAL) = 7761845
XdndAware(ATOM) = BITMAP
_NET_WM_ICON(CARDINAL) = Icon (128 x 128): [Icon here]
_NET_WM_OPAQUE_REGION(CARDINAL) = 0, 0, 1920, 992
_NET_WM_WINDOW_TYPE(ATOM) = _NET_WM_WINDOW_TYPE_NORMAL
_NET_WM_SYNC_REQUEST_COUNTER(CARDINAL) = 186646546, 186646547
_NET_WM_USER_TIME(CARDINAL) = 7777827
_NET_WM_USER_TIME_WINDOW(WINDOW): window id # 0xb200011
WM_CLIENT_LEADER(WINDOW): window id # 0xb200001
_NET_WM_PID(CARDINAL) = 16411
WM_LOCALE_NAME(STRING) = "de_DE.UTF-8"
WM_CLIENT_MACHINE(STRING) = "Bastis-ThinkPad"
WM_NORMAL_HINTS(WM_SIZE_HINTS):
                program specified minimum size: 584 by 210
                program specified base size: 0 by 0
                window gravity: NorthWest
WM_PROTOCOLS(ATOM): protocols WM_DELETE_WINDOW, WM_TAKE_FOCUS, _NET_WM_PING, _NET_WM_SYNC_REQUEST
WM_CLASS(STRING) = "xournalpp", "Xournalpp"
WM_ICON_NAME(STRING) = "Ungespeichertes Dokument - Xournal++"
_NET_WM_ICON_NAME(UTF8_STRING) = "Ungespeichertes Dokument - Xournal++"
WM_NAME(STRING) = "Ungespeichertes Dokument - Xournal++"
_NET_WM_NAME(UTF8_STRING) = "Ungespeichertes Dokument - Xournal++"

Contents of the .desktop file named com.github.xournalpp.xournalpp.desktop:

[Desktop Entry]
Type=Application

Name=Xournal++
Comment=Take handwritten notes
Keywords=Tablet;Wacom;Pen Input;PDF Annotation;Markup
Name[fr]=Xournal++
Comment[fr]=Prise de notes manuscrites
Name[ca]=Xournal++
Comment[ca]=Preneu notes a mà
Name[cs]=Xournal++
Comment[cs]=Dělejte si rukou psané poznámky
Name[de]=Xournal++
Comment[de]=Handnotizen erstellen
Name[pl]=Xournal++
Comment[pl]=Notatki odręczne
Name[zh_CN]=Xournal++
Comment[zh_CN]=手写注释工具，可为PDF文件添加各种元素的注释标记
Name[ru]=Xournal++
Comment[ru]=Делайте рукописные заметки

Exec=xournalpp %f
StartupWMClass=xournalpp,Xournalpp,Xournal++
Terminal=false
StartupNotify=true
MimeType=application/x-xoj;application/x-xojpp;application/x-xopp;application/x-xopt;application/pdf;
Icon=com.github.xournalpp.xournalpp
Categories=Office;GNOME;GTK;

I also tried to reach out to the developers of Xournal++ which unfortunately couldn't help me and suspected a bug within KDE, maybe related to cache. Please see https://github.com/xournalpp/xournalpp/issues/3289

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: plasma-workspace 4:5.22.5-0ubuntu1
ProcVersionSignature: Ubuntu 5.13.0-22.22-generic 5.13.19
Uname: Linux 5.13.0-22-generic x86_64
ApportVersion: 2.20.11-0ubuntu71
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: KDE
Date: Fri Dec 3 12:19:44 2021
ExecutablePath: /usr/bin/plasmashell
InstallationDate: Installed on 2020-09-10 (449 days ago)
InstallationMedia: Kubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
ProcEnviron:
 PATH=(custom, user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
 LANGUAGE=de
SourcePackage: plasma-workspace
UpgradeStatus: Upgraded to impish on 2021-10-20 (43 days ago)

[Scarlett Gately Moore (scarlettmoore)]

This bug is very old, if it is still an issue please update the bug with info from a supported release.