After negating weights, a positive cycle in the original graph becomes a negative cycle in the converted graph. A positive cycle is a loop where you gain score by traversing it. Bellman-Ford will detect this negative cycle in the converted graph, which means you can achieve infinite score in the original problem by looping through the cycle.
But you also need to check if the cycle is reachable from room and if room is reachable from the cycle. If the cycle is not on any path from to , it does not affect the answer.