Methods to calculate protein content of foods have been discussed for many years, with nitrogen conversion factors (NCF) acknowledged to be estimates that may lead to 15-20% error in estimation of protein content. This has led to more accurate NCFs, based on newer data and analytical methods, being published yet not widely adopted.
Food composition data compilers may be interested to watch the series of talks, on the subject, presented at the EuroFIR/INFOODS Protein calculation workshop held earlier this month https://www.eurofir.org/news-and-events/12191-2/meetings-and-events/