juju upgrade-juju yields "no upgrades available"

I have a working juju environment running juju 1.20.14 (which was upgraded from juju 1.18).

I want to upgrade the environment to latest 1.24.

First I went on upgrading the juju client:

$ sudo apt-get upgrade
sudo: unable to resolve host mariosplivalo-bastion
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages will be upgraded:
  juju-core
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/37.8 MB of archives.
After this operation, 231 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
(Reading database ... 83950 files and directories currently installed.)
Preparing to unpack .../juju-core_1.24.4-0ubuntu1~14.04.1~juju1_amd64.deb ...
Unpacking juju-core (1.24.4-0ubuntu1~14.04.1~juju1) over (1.18.1-0ubuntu1) ...
Setting up juju-core (1.24.4-0ubuntu1~14.04.1~juju1) ...
update-alternatives: using /usr/lib/juju-1.24.4/bin/juju to provide /usr/bin/juju (juju) in auto mode

After that I wanted to upgrade juju:

$ juju upgrade-juju
no upgrades available

However, when I specify the version I want to upgrade to the upgrade process commences:

$ juju upgrade-juju --version=1.24.4
available tools:
    1.24.4-precise-amd64
    1.24.4-precise-armhf
    1.24.4-precise-i386
    1.24.4-trusty-amd64
    1.24.4-trusty-arm64
    1.24.4-trusty-armhf
    1.24.4-trusty-i386
    1.24.4-trusty-ppc64el
    1.24.4-vivid-amd64
    1.24.4-vivid-arm64
    1.24.4-vivid-armhf
    1.24.4-vivid-i386
    1.24.4-vivid-ppc64el
best version:
    1.24.4

Although it's not clear from the output the upgrade procedure started and it eventually finishes without an issue.