- Plesk Onyx 17.8
- Additional Services were modified in Service Plans > Additional Services under Plesk Administrator.
- Only WordPress Toolkit and Let's Encrypt services persist in Service Plans > Add a Plan > Additional Services under the reseller account.
This is Plesk bug with ID #PPPM-8390 that is planned to be fixed in future Plesk updates.
Currently, there is no workaround available.