Missing panel icon for specific application (Session Private Messenger)

Hello, I was advised to create a bug report after opening a thread about this issue.

I am using Manjaro 21.1.6 with xfce4-panel 4.16.3 and xfce4-whiskermenu-plugin 2.6.0. Session Private Messenger is the latest available version at this time, 1.7.3. The same issue has been present when using the Flatpak, the Snap, the AppImage from Session's website and the session-desktop-bin AUR package. I tried to compile from source but kept running into errors with that so I'm not sure if that would also have the same outcome.

It looks like this. Icons for other applications all appear fine. Using different style/icon sets had no effect. I tried the steps recommended by ToZ:

#!/bin/bash
ICON=/var/lib/flatpak/app/network.loki.Session/current/active/export/share/icons/hicolor/512x512/apps/network.loki.Session.png

WINDOWS=(`wmctrl -l | grep "$hostname Session" | cut -f1 -d' ' | xargs`)
for window in ${WINDOWS[@]}; do
    # Use "xseticon", a compiled C binary, to change the icon of a running program
    /usr/bin/xseticon -id ${window} $ICON

    # Use "xprop" to set the window state, so that alt+tab works again
    xprop -f _NET_WM_WINDOW_TYPE 32a -set _NET_WM_WINDOW_TYPE _NET_WM_WINDOW_TYPE_NORMAL -id ${window}
done

But that is unfortunately met with an error: Aborted at function fopen()

Any help would be greatly appreciated, thank you so much.