Ansible is using the Python lib socket to fill the fqdn and domain value in the facts (ansible_facts).
But for the hostname, it uses the lib platform.
And of course, as their names suggest it, they are retrieving the data with different approach:
The lib socket can sometimes be weird!
In a corner case I encountered (see also https://bugs.python.org/issue5004):
the domain value in ansible_facts was empty and thus unexpected behaviours occured. The lib socket was obviously lost…
To avoid such problem, the solution is to rely only on the lib platform.
So replace your
ansible_facts['nodename'].split('.')[1:] | join('.')