Public–private partnerships (PPP) are often used to expand water supply systems (WSS), but success depends on more than building treatment and transmission facilities. This paper explains why performance can still fall short after operations begin when downstream readiness, completed distribution pipes, enough household connections (HC), and reliable metering and billing do not keep pace with the planned service rollout. The study examines the Bandar Lampung project through a documentary single-case approach, relying on official project records to track how gaps in downstream readiness affect real service delivery and financial stability. The evidence shows that household connections were delivered far below planned levels, while parts of the downstream network remained incomplete. As a result, bulk water could not be absorbed at the expected scale, weakening the chain from supply to paying customers and increasing financial pressure on the public side. This pressure was reflected in the recorded utilization of government guarantee support during operations. The paper interprets the problem as a practical interface gap in a split-scope arrangement: one party delivers upstream capacity, while another controls the downstream steps needed to turn capacity into paying demand. The paper proposes monitorable safeguards, readiness gates, phased activation by service zones, clear responsibility maps, early-warning indicators, and protected downstream funding triggers to reduce the chance that service gaps escalate into disputes and contingent liabilities. The most important consequence is that downstream readiness must be managed as a verifiable obligation, not assumed once facilities are built.