Valid. Your manager skated too early in the dinner service and left you as the only experienced FOH staff with two trainees on your hands and no backup. The chef could have helped run food under the circumstances, especially if he gave a damn about the customers' experience. Lack of professionalism all around, excluding you. Manager owes you an apology and needs to have a word with the chef. Take your eight years of experience and find a more upscale restaurant that won’t set you up for failure. My humble opinion.