Thank you! I think I've figured it out. In the pods section, two “Contract Satellit Bus” parts appear, Nav Sat and Comm Sat, respectively. This is where the payload is loaded.
Somehow I didn't notice this before. I also played RP-1 v1.0 on version KSP 1.10.1. For old times’ sake, I was looking for a payload in service modules.
When I encountered my problem. Internet searches yielded no results. Even in the FAQs section on Github in paragraph 4. Completing Contracts, I understand that we are talking about tanks. XD