And, are we talking about a person's taste perception (my step-daughter doesn't like hamburger buns with sesame seeds on them because she thinks they taste bitter - I don't taste that) - or are we talking about tahini that has oxydized and gone "rancid" - which all oils will do, peanut butter, tahani, vegetable oil, Crisco ... etc. ???
Can the OP differentiate between "bitter" and "rancid"? Can you describe how to tell the difference? In my experience - rancid is more of a rotten and "sour" taste - and it also has an off "odor" - bitter is odorless and somewhat subjective.
I don't know if the OP was tasting what might be the bitterness that some people taste - or maybe it was just starting to go rancid ...
Oh - and getting tahani from a middle eastern store is no guarantee ...
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain