Explanation
According to the PMBOK Guide, 7th edition, the project manager should engage stakeholders to understand their needs and expectations, and to manage changes that may affect the project scope, schedule, cost, quality, or benefits1. In this case, the missing regulatory feature is a change that may have significant impact on the project objectives and constraints, especially the fixed release date. Therefore, the project manager should invite stakeholders to discuss the impact of adding the feature, and to agree on the best way to handle it, such as reprioritizing the backlog, adjusting the iteration length, or negotiating the release date. This is also consistent with the agile principle of customer collaboration over contract negotiation2. References:
1: PMBOK Guide, 7th edition, Chapter 3: Engage Stakeholders, p. 51-52
2: Agile Practice Guide, p. 9