Modular electrolysis plants can provide flexible, price-responsive hydrogen production, but coordinating multiple electrolyzer units under volatile operating conditions and electrolyzer failures remains challenging. This work reports on laboratory experiments with a modular electrolysis plant demonstrator comprising three anion-exchange membrane electrolyzers, operated using a multi-agent real-time optimization approach that minimizes electricity procurement costs on a rolling horizon. Three test cases are evaluated: price-responsive operation under a static demand target, operation under electrolyzer failures, and a combined scenario with demand changes and failures. Across all scenarios, load is shifted toward low-price periods. Under nominal conditions, cumulative production deviates by less than one percent from the target. Under electrolyzer failures, load is redistributed to the remaining units within their admissible operating ranges. The results provide proof-of-concept evidence that modular electrolysis plants, coordinated via standardized interfaces and decentralized optimization, can operate as flexible assets in hybrid energy systems.