Honestly, no idea. We are using a library for implementing tray support for the client. It seems your desktop environment does not offer the required interfaces for the tray library that the r3 client uses.
Here are some notes to the topic from the developers Github repo:
Linux/BSD
This implementation uses DBus to communicate through the SystemNotifier/AppIndicator spec, older tray implementations may not load the icon.
If you are running an older desktop environment, or system tray provider, you may require a proxy app which can convert the new DBus calls to the old format. The recommended tool for Gnome based trays is snixembed, others are available. Search for "StatusNotifierItems XEmbedded" in your package manager.