I recently had the following issue in vCenter : A non-existent network still associated with a virtual machine but marked as inactive. The VM has no longer any network interface mapped to that network but it still show in the network map and the VM overview.
It turned out that a snapshot for this VM taken a while ago when that network was mapped to this VM was still existing in the snapshot manager.
This is not a bug, it will remain as long the snapshot exist.
To “fix” this, you have to remove the old snapshot, otherwise you can leave it as is without any issue.