When an RDX storage is used as a repository in Veeam Backup & Replication, and a backup job from Veeam Agent for Microsoft Windows is targeted at it, a new backup chain will start each time the RDX cartridge is changed.
The behavior is by design. More information is available in the User Guide.
If the RDX storage is presented to Veeam Agent for Microsoft Windows directly (not as a Veeam Backup & Replication repository), each cartridge will contain its own backup chain.
When a cartridge is injected for the first time, a new chain will start. When a cartridge with an existing chain is injected, the chain stored on this specific cartridge will resume.