This occurs on the class level. Hosts, hostgroups and nested hostgroups appear to have the correct behavior. This occurs in the nightly (October 7, 2015) installed with a default foreman-installer setup, as well as in version 1.9.2.

When adding a parameter override of type array, yaml or hash that has an element which is an interpolated erb value, the first time one clicks submit, the values appear correctly.

Open the class again and you will see that the parameter overrides have been mangled. Click submit, and the yaml values for the host will show true for arrays and hashs, and will be prefixed by multiple instances of the following (and it gets longer each submission):
! '--- ! "---\r\n