If a single node (using loopback address ::1) cluster has been configured in IPv6 mode in 5.5.0 and an attempt is made to upgrade the cluster to later versions, then all data will be lost. This is because the cbupgrade script doesn't handle IPv6 loopback address correctly. The reason for this is that the script can't determine if the cluster has been operating in IPv6 mode as the static_config would've got overwritten as part of the upgrade.
Note that this problem will not be seen on a single node cluster where the node was configured with an FQDN. Since single node cluster configured in IPv6 mode using loopback address will not be a production cluster, we've decided not to fix this but to document the behavior.