Actually it returns the IP allocated via DHCP, however the ip-normalize script needs a prefix for the regex, which is not set when dhcp was set.

I did some further investigation on this topic, and it turned out that this is a more general problem, which does not just happen in the specific case I wrote above.
It basically happens anytime when an interface is first configured via DHCPv4 and then DHCPv4 is disabled later.

Easier way to reproduce:

Activate DHCP for a given interface

delete interfaces ethernet eth0 address
set interfaces ethernet eth0 address dhcp
commit

De-activate DHCP

delete interfaces ethernet eth0 address
set interfaces ethernet eth0 address dhcp
commit

Resulting error message:

Traceback (most recent call last):
  File "/usr/libexec/vyos/system/normalize-ip", line 32, in <module>
    address_string, prefix_length = re.match(r'(.+)/(.+)', sys.argv[1]).groups()
AttributeError: 'NoneType' object has no attribute 'groups'

The problem is:
The file "/opt/vyatta/sbin/vyatta-address" calls "/usr/libexec/vyos/system/normalize-ip" with the IP address.
In case of a DHCPv4 configured interface, the IPv4 address is parsed from the dhclient lease file and then passed to the "normalize-ip" script.
That script expects an address with network prefix in CIDR notation, which is not provided here (only the pure IPv4 address is passed).

This may also apply to DHCPv6.

This problem is fixed by the following pull request:
https://github.com/vyos/vyatta-cfg-system/pull/89

Ooops X-fire. I grab the IP and prefix from the interface, that way I know it exists and can be removed and won't have to many test cycles. The subsystem needs to be rewritten at one point, but as you can imagine that is quite a task. DHCPv6 won't be affected since it uses always IP/prefix, but I haven't tested it yet.