I've had pure stem and vein from a few very reliable sources, several years ago before the great kratom quality crash of the 2020s I wouldn't describe it as potent as hell, but it is certainly psychoactive. Probably 50-75% less potent than kratom leaf, iirc. IDK, it's been awhile. Some vendors mix it with leaf itself then label it as stem and vein.
Anyways, it sure is a quality issue. But from what? Bad rainy season? Toxins in the soil? Improper drying/storage/processing?
It's not age or oxidation, I know that.
What I have noticed is that kratom that tastes unusually bitter relative to normal tends to have the wobble issue... but not always. Also, this seems to be an issue with people who move/buy weight in large scale wholesale.
Bit of a mystery.