From time to time, you can see that an application node belonging to one of your instances will show offline for several days within the System Diagnostic page.
The cause of why the node appears offline within the System Diagnostics page for several days is due to it being retired and replaced by a new node.
The offline application node will be removed from the System Diagnostics page after seven days or more. There is an internal backend job that runs to check when the node has been last communicated with and if the amount of time is more than seven days, then it will be removed.
You can check how long an application node has before it is removed by navigating to [sys_cluster_state] table and searching for the application node in question. From there, you can open up the corresponding record and check when the 'Most recent message' has been. This is the field that is used for calculation on when this record should be removed.