This is expected agent behavior. VCS assumes that data is being read from or written to the volumes and does not declare the resource as offline. This prevents potential data corruption that could be caused by the disk group being imported on two hosts.
You can deport a disabled disk group when all I/O operations are completed or when all volumes are closed. You can then reimport the disk group to the same system. Reimporting a disabled disk group may require a system reboot.