Description of problem:
I'm not sure if this is a bug, but it seems to be one.
Steps to Reproduce:
1. create host
2. create network
3. in setup host networks assign network
4. in cluster's manage network uncheck assigned and requested.
5. in setup host networks there's unmanaged network, remove it.
6. reenable assigned and required checkboxes for given network in cluster's manage network.
host is up after that
host should be not functional since it does not have required network assigned.
This sounds like correct behavior - once the network is detached from the cluster and then re-attached, it is principally a new network. A new network, even if it's required, will not render hosts non-operational - instead the network itself will appear as non-operational (red down arrow) until it's configured on all active hosts. Only from that point on specific hosts will move to non-operational state if the network is missing on them.