Release 21.0.1 is the latest stable release of OpenNMS. It contains a few enhancements but is primarily a bugfix release, including a fix for the rendering issue in the topology UI and other Vaadin-based UI elements.
For a high-level overview of what’s changed in OpenNMS 21, see What’s New in OpenNMS 21.
For a complete list of changes in 21.0.1 see the release notes.