By Default Spectrum does not automatically remove connections once a known device is no longer on the network. This is due to Spectrum does not know if the device was down or removed, and as Spectrum would normally alarm on a down device, it will not remove the connection if the device is lost contact.
Release : 10.4
Component : Spectrum CORE
Out of Box, Spectrum automatically reconfigures interface models. When the interface is reconfigured, the IP tables are read once more and connections are resolved. In some cases if Spectrum cannot resolve the connections, or if automatic reconfigure of interfaces has been disabled due to performance issues, Spectrum may not be resolving the connections automatically. In these cases it may be necessary to manually reconfigure devices to determine proper connectivity.
You can either manually delete the old connections, or manually reconfigure the device. Please see the documentation for more detail:https://techdocs.broadcom.com/content/broadcom/techdocs/us/en/ca-enterprise-software/it-operations-management/spectrum/10-3-2/managing-network/modeling-and-managing-your-it-infrastructure/update-device-interface-and-connection-information.html