Just verified that an upgrade from 1.16.0 to 1.16.1 doesn't convert hashes.
Therefore it is likely that the issue was introduced in the version 1.17.

It is likely that this "bug" was introduced by PR #5403 (https://github.com/theforeman/foreman/pull/5403).
Should all YAML lookup_values become JSON hashes? Especially since it is possible to change the values back to YAML hashes (through the UI) and they get written like that to the DB.

Hello,

https://github.com/theforeman/foreman/blob/1.17-stable/db/migrate/20170112175131_migrate_template_to_parameters_macros.rb#L34

parameter.value is a ruby Hash when Lookup_value type is Yaml/Hash in Foreman.
You convert it in String with to.s.
But you don't convert this String in Hash before update_attribute.
So YAML content is replaced by the String 'ruby Hash'.

Same problem with Array in Foreman (it's a ruby Array).
And same problem for LookupKey.default_value (https://github.com/theforeman/foreman/blob/1.17-stable/db/migrate/20170112175131_migrate_template_to_parameters_macros.rb#L25).