Removing node state will not be possible when the node is a seed node since Service Fabric 6.5. It will be necessary to convert the seed node into a non-seed node prior to node state removal. It is not yet known how to convert seed nodes into non-seed nodes. It will be possible to opt out of this limitation in the cluster configuration. Detailed guidance will be available when Service Fabric version 6.5 is released.
Release notes explain the reason for this change:
A Service Fabric cluster requires a quorum of seed nodes to be up. If a quorum of seed nodes is not up, the cluster goes down. Node state removal erases the node from the databases of all Service Fabric system services, including the health management service. As a result, when the node state is removed, the health management service removes all the error events associated with that node. This includes the error event which previously notified the user that a seed node in the cluster is down. Therefore, the down seed node remains unnoticed and the cluster appears healthy even though it is not. For this reason, node state removal of seed nodes will be disallowed.