Strike legality; mandatory strike vote and notice requirements practice question

The United Health Workers Union is the certified exclusive bargaining agent of rank-and-file employees at City General Hospital. After the hospital management repeatedly refused to negotiate a collective bargaining agreement, the union sent several letters requesting negotiations, which the hospital ignored. On November 1, 2022, the union filed a notice of strike with the National Conciliation and Mediation Board (NCMB), citing management's refusal to bargain. On November 5, 2022, the union leadership claims to have conducted a strike vote by secret ballot at the hospital parking lot, with a majority voting in favor of a strike. However, the union did not notify the NCMB at least 24 hours before holding the strike vote, nor did it submit a report of the results to the NCMB before staging the strike on November 10, 2022. The hospital asserts that no valid strike vote occurred and that the strike is illegal due to non-compliance with legal requirements. The union counters that the hospital's unfair labor practices justified their actions. Is the strike staged by the union legal? Explain.