salt: Failed to load ext_pillar stack: no filter named 'unique' in 2018.3

Description of Issue/Question

After upgrading to 2018.7 Saltstack doesn’t know the Jinja filter ‘unique’ in the ext_pillar stack. This worked ok before upgrading from 2017.7 and after downgrading back to 2017.7.

local:
    Data failed to compile:
----------
    Pillar failed to render with the following messages:
----------
    Failed to load ext_pillar stack: no filter named 'unique'

Setup

snippet from external pillar stack *.yml file:

{% for app in a_dict | unique %}
{% endfor %}