xfce4-panel window buttons: No icon for Slack

In xfce4-panel 4.20.4, Fedora 42, the Slack application icon is rendering as a black square. How can I troubleshoot this? This is in a fresh install of Fedora 42. Everything worked as expected on my other machine's Fedora 40 installation. I've never seen this square icon behavior in older XFCEs.

All the other application icons render okay. I tried changing the slack.desktop to explicitly point to where the rpm-installed Slack icon is (/usr/share/pixmaps/slack.png, as expected):

eric@lat:~$ locate slack.desktop
/home/eric/.config/autostart/slack.desktop
/usr/share/applications/slack.desktop

eric@lat:~$ cat /home/eric/.config/autostart/slack.desktop
[Desktop Entry]
Name=Slack
StartupWMClass=Slack
Comment=Slack Desktop
GenericName=Slack Client for Linux
Exec=/usr/bin/slack %U
Icon=/usr/share/pixmaps/slack.png
#Icon=slack (commenting this made no difference)
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Network;InstantMessaging;
MimeType=x-scheme-handler/slack;

eric@lat:~$ file /usr/share/pixmaps/slack.png
/usr/share/pixmaps/slack.png: PNG image data, 512 x 512, 8-bit/color RGBA, non-interlaced

The Slack status icon renders as expected in the Status Tray plugin. Both Window Menu and Window Buttons render the empty square, instead of this icon: