Multiple node attachment action for a cluster fails partially if there is a node in an offline state within the batch of nodes to be added. The legacy U/I gives a "404 Not Found" error and stays on the same "Add Nodes" tab. In the new U/I, the same error is given, but it navigates back to the Infrastructure Page.
Platform9 Managed Kubernetes - v4.5 and Lower
Product bug UX-675 has been filed to investigate and rectify the issue and will be fixed in the upcoming releases.
One of the ways to work around the issue is to selectively choose the nodes that you want to add to the cluster having prior knowledge of which one is in an offline state.