Sorry to bring back this older thread - I noticed a slight improvement of behaviour in more recent versions of ROS, don’t know exactly it started but at least in 7.18 the address prefix seems more predictable after a reboot.
It seems to me that the delegated prefix bits are now assigned in order based on the interface sorted alphabetically.
I haven’t been able to test this too extensively except the several reboots post ROS upgrade, but at least my Network ID bits is idempotent between reboots.
Ideally I’d still want more control over the Network ID bits, so there isn’t a linked-list effect upon reboot, but at least it is more predictable than before.