Losing ingress on a single broker can bring partitions under min ISR and even offline if this broker is restarted.

This is because when ingress is lost, the broker kick all consumers from their groups it's coordinating (as they can't heartbeat anymore). This causes a write to the associated __consumer_offsets partition to mark the groups empty. As broker can't replicate that data, the broker marks them out of sync and the partitions end up with a single ISR.