I just got the plugin working and logged an account in. In Account > Spam Protection, the Policy is set to Non-Paying. In ISPConfig the mail domain is set with policy Normal, and accounts use "- Inherit domain setting -". Set the policy in Roundcube and it does go through the API to properly set the policy for the mail user. Set the policy in ISPConfig to one of the specific options and it does show the change in Roundcube.
The problem is that "- Inherit domain setting -" isn't in the list in the Roundcube UI, so it can't be selected in the dropdown and the selection can't be highlighted in the Available Policies table. Verify by changing the policy for the mail user back to "- Inherit domain setting -", then go back to the Roundcube UI, and once again it's unset.
[EDIT] I diagnosed this incorrectly when I first created the ticket. Edited a couple times to get it right.