Icons not showing in Workspace Switcher

I have a panel set up to look like this:

However, on upgrading to xfce4-panel-4.18.1-1, the icons in the Workspace Switcher disappear for no obvious reason. Downgrading to xfce4-panel-4.18.0-1 makes this issue go away.

I am using EndeavourOS, with xfce4-panel from the Arch Linux extra database; my window manager is XMonad, but otherwise the rest of my desktop uses Xfce.