As per the title, the options for the _ANSIBLE_TEMPLAR_UNTRUSTED_TEMPLATE_BEHAVIOR as defined here are listed as

warning - A warning will be issued when an untrusted template is encountered.

fail - An error will be raised when an untrusted template is encountered.

ignore - Untrusted templates are silently ignored and used as-is. This is the default behavior.

But fail should be error instead, as can also be seen in the mentioned file and the error message when choosing fail as value for said env variable.