I sometimes come across apples that taste very bitter (not sour like a granny smith, but more like tannin). This is maybe one in 10 or 15 apples. It doesn't seem to be restricted to a certain variety (I've tasted it in pink lady, opal, honeycrisp and some I've forgotten). Sometimes it's just one, and sometimes it's a whole batch. Washing doesn't remove the flavor
Google search suggestions show other people have noticed this too, but I can't find a good answer as to why. It seems too common to me to not have a good answer though; maybe it's something only some people can taste, like cilantro tasting soapy or cyanide smelling like almonds?