We are going to fix the bug PPPM-4473 in Plesk Onyx in one of the upcoming updates.
And also you can get updated Plesk provisioning module on WHMCS side when WHMCS will provide update to version 7.1.0.
Plesk 17.8 makes a number of backwards incompatible changes to the API. As such the module for WHMCS is not currently fully compatible. We suggest operating Plesk 17.5 at this time.