A reseller copied provider's hosting plan (
Plesk Client) with Create a copy of existing Provider's hosting plan.
Subscriptions based on the hosting plan are provisioned with provider's name server even though separate name servers set is created for the reseller.
If reseller's plan is copied -- provision is done with provider's settings (provider's name servers). If reseller's plan is their own (made from scratch in rcc) -- domains are provisioned with reseller's name servers.
Create a hosting plan for the reseller (not copy), so that PBA-S uses reseller's name servers.