Some hubs, when red for a period of time, do not disappear. Others do. Seems to be tunnel hubs that disappear.
Hubs with direct connection (proximity 0) will not be removed from Infrastructure Manager if they are down for an extended period of time -- they will still show red.
But, for non-directly connected hubs (tunnels), after a default interval of 1800 seconds they will disconnect. This default interval can be increased or decreased by adding the following key/value pair in hub section in hub.cfg:
The value is in seconds.
Release: CNMSPP99000-8.4-Unified Infrastructure Mgmt-Server Pack-- On Prem