playbook variables defined in includes from vars_files: not usable throughout the playbook., regression from 1.7.2 to 1.8.2

The almost minimal example below works fine in ansible 1.7.2 but does not give same and expected result in 1.8.2.

In short what this script is doing is to increment a counter in the hosts_names.yml file whenever the variable system_upgrade equals ‘True’.
This works fine with ansible 1.7.2 but on 1.8.2 a default value I use for the variable that should be used if the file hosts_names.yml doesn’t exist partly take precedance over the value read from the file.

The playbook is invoked with eighter of these two command lines:
ansible-playbook sample.yml -u msp --extra-vars “userid=msp system_upgrade=False”
ansible-playbook sample.yml -u msp --extra-vars “userid=msp system_upgrade=True”

If someone interested I can supply a tar file with the complete sample project.