In short, the answer is that it entirely depends upon the ingredients that comprise the Plant Protein Powder.
The misconceptions arise due to the heavily concentrated chemical-laden protein powder brands available in the market, known as Plant Protein isolate and Plant Protein concentrate. These are generally made from Rice and Peas, (although peas do have some amount of Zinc, rice does not).
These products due to marketing gimmicks try and prove that only these powders have protein.
Well, the definition of Protein is short and Simple– "Any food that has nine essential amino acids" is Protein.
Since food from animal sources like milk, chicken, eggs, and fish already have these nine essential amino acids, the misconception is further aggravated by stating that plant sources are not complete proteins.
Yes, Most plant sources lack one or two essential amino acids, but a combination of two or more vegetarian sources completes the amino acid profile. For example a combination of Almond and Garbanzo beans, a combination of rice and peas, or a combination of Almond and flaxseed, and similarly there are numerous other combinations.