Changing a hosts hostgroup (by using the host edit page) converts a smartclass YAML value to a string.

Before:
yaml_array:
- stuff
- more_stuff

After changing the hostgroup:
--- "yaml_array:\r\n- stuff\r\n- more_stuff\r\n"

It seems like that this only happens when using the hosts edit page directly.
The YAML value stays intact when changing the hostgroup by using the button "Select Action" from the "Hosts -> All Hosts" page ...