Steps to reproduce:

1. Create a template with discovery rule with host prototype and link this template to some host.
2. Add a custom host interace to templated host prototype using hostinterface.create API method.

Result:
The host interface was added only to templated host prototype. The same host interface for inherited host prototype was not added.
Expected:
The host interface was added as for templated host prototype, as for inherited.

The same issue happens using hostinterface.update, hostinterface.delete API methods.