I can confirm this happens:

root@test:~# apt-cache policy python-nova
python-nova:
  Installed: 2:13.1.4-0ubuntu4.5
  Candidate: 2:13.1.4-0ubuntu4.5
  Version table:
 *** 2:13.1.4-0ubuntu4.5 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status
     2:13.0.0-0ubuntu2 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages
root@test:~# apt-cache policy python-oslo.middleware
python-oslo.middleware:
  Installed: 3.8.0-2ubuntu1
  Candidate: 3.8.0-2ubuntu1
  Version table:
 *** 3.8.0-2ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status
     3.8.0-2 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages
root@test:~# add-apt-repository cloud-archive:ocata
<--snip-->
root@test:~# sudo apt install python-nova
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  python-ndg-httpsclient
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  alembic os-brick-common python-alembic python-babel python-babel-localedata python-blinker python-ceilometerclient
  python-cffi python-glanceclient python-jwt python-keystoneauth1 python-microversion-parse python-oauthlib python-os-brick
  python-os-vif python-os-win python-os-xenapi python-oslo.concurrency python-oslo.config python-oslo.context python-oslo.db
  python-oslo.log python-oslo.messaging python-oslo.policy python-oslo.privsep python-oslo.service python-oslo.utils
  python-oslo.versionedobjects python-osprofiler python-ply python-positional python-pycparser python-pyparsing
  python-requests python-stevedore python-tenacity python-urllib3 python-webob
Suggested packages:
  python-editor python-blinker-doc python-ceilometerclient-doc python-dev python-glanceclient-doc python-keystoneauth1-doc
  python-requests-kerberos python-microversion-parse-doc python-ldap python-os-brick-doc python-os-vif-doc python-os-win-doc
  python-os-xenapi-doc python-oslo.concurrency-doc python-oslo.log-doc python-kafka python-zmq python-oslo.policy-doc
  python-oslo.privsep-doc python-oslo.service-doc python-oslo.versionedobjects-doc python-ply-doc python-positional-doc cpp
  python-pyparsing-doc python-socks python-tenacity-doc python-ntlm python-webob-doc
The following NEW packages will be installed:
  python-blinker python-ceilometerclient python-cffi python-jwt python-microversion-parse python-oauthlib python-os-vif
  python-os-xenapi python-oslo.privsep python-osprofiler python-ply python-pycparser python-tenacity
The following packages will be upgraded:
  alembic os-brick-common python-alembic python-babel python-babel-localedata python-glanceclient python-keystoneauth1
  python-nova python-os-brick python-os-win python-oslo.concurrency python-oslo.config python-oslo.context python-oslo.db
  python-oslo.log python-oslo.messaging python-oslo.policy python-oslo.service python-oslo.utils
  python-oslo.versionedobjects python-positional python-pyparsing python-requests python-stevedore python-urllib3
  python-webob
26 upgraded, 13 newly installed, 0 to remove and 38 not upgraded.
Need to get 8449 kB of archives.
After this operation, 15.5 MB of additional disk space will be used.
Do you want to continue? [Y/n]
<--snip-->
root@test:~# apt-cache policy python-nova
python-nova:
  Installed: 2:15.1.5-0ubuntu1~cloud5
  Candidate: 2:15.1.5-0ubuntu1~cloud5
  Version table:
 *** 2:15.1.5-0ubuntu1~cloud5 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/ocata/main amd64 Packages
        100 /var/lib/dpkg/status
     2:13.1.4-0ubuntu4.5 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
     2:13.0.0-0ubuntu2 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages
root@test:~# apt-cache policy python-oslo.middleware
python-oslo.middleware:
  Installed: 3.8.0-2ubuntu1
  Candidate: 3.23.1-0ubuntu1.1~cloud0
  Version table:
     3.23.1-0ubuntu1.1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/ocata/main amd64 Packages
 *** 3.8.0-2ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu xenial-security/main amd64 Packages
        100 /var/lib/dpkg/status
     3.8.0-2 500
        500 http://archive.ubuntu.com/ubuntu xenial/main amd64 Packages