In an IBFT2 network containing:
4 Validators (all of which are not bootnodes)
When one validator was terminated, and restarted, when it queried the bootnodes at startup only the bootnodes were reported as neighbours - the other 3 validators were not reported.
Ethereum network traffic reached all nodes in the network, and the chain could be seen to be progressing (but at a staggered cadence, as the restarted validator effectively appeared to be not be participating in IBFT2 as it did not have a connection to any of the other validators).
When an admin_peers was executed against the bootnodes, they reported each other and the restarted-node, but NOT the 4 validators.
I.e. it appears the 3 non-restarted validators were not appearing (or not bonded?) in the bootnode peer-table - but they were able to communicate.
and saw this in action so are the people to get in touch with for more information.