I don’t know these games (I’m out of the loop on commercial and board games for the past 20-odd years, sadly) but I have loved this mechanism since playing the earliest rummy-family games from China. (Majiang is the most recent of a loooooooooooooooooong line of build-and-discard games in China.) The added spice of worrying if someone is going to use your discard to improve their hand or even complete it before you can do yours is part of why I like playing the rummy family.
B12 is prevalent in the diet, but not everybody has the so-called “intrinsic factor” that permits the body to actually process it. If you lack it it’s very difficult to get enough B12 no matter what you eat. (Ask me how I know…)