Information regarding Brassica juncea var. rugosa (芥菜) and it's bitter leaves
Bolted?
This is a mustard related to bok choi, napa cabbage etc.  If mustard (or many other leafy plants) bolt and set flowers they can become bitter.  One would think that they would not then be sold, but sellers can cut off the flowers and hope no-one notices.  I once bought some like this at the farmers market and a different seller caught sight of it and pointed out where the flowers had been cut off.
I am not super averse to bitter so I ate it anyway.