Performing a port-update with a different set of fixed-ips than are currently on the port will be reported as a success by Neutron, however the actual addresses will not be updated in the Linux network namespace. This now has more functional implications as a result of multiple subnets being allowed on the external router interface (https://review.openstack.org/#/c/149068). If the interface has two subnets and the user wishes to remove one, they will have to clear the gateway interface first, removing both (causing traffic disruption), delete the subnet, and re-set the gateway on the router to re-add the remaining subnet. If port-update were functional for router addresses, this command could be used to remove a second subnet without causing disruption to the first.
This bug has not been updated by the assignee for a year - so unless there is any update let's allow this to expire.