File Operation tray icon not disappearing

With Thunar 4.16.10 and xfce4-panel 4.16.4, here is part of my panel. Look at the tray icons:

No, there aren't any file operations in progress. Tapping on any of the Thunar icons reveals:

It's resizable, and remembers its dimensions upon returning to.

On one hand, it could be an issue with xfce4-panel:

• This does not happen on another PC with identical configuration, except for being connected to an external display only, while this PC here is using its internal display only.
• This started happening around the same time as xfce4-panel is updated.

On the other hand, it could be an issue with Thunar:

• Toggling the tray icon in Gpick works fine.
• Killing Thunar destroys its icons.

The Thunar instance is fresh, nothing is printed onto the console.

You can try to reproduce the above by making copies of many empty files.