If a
synthetic food additive were that hazardous at that small a multiple of the amounts used in food, it'd have been banned. This (curcumin) is an exemplar of that pervasive magical thinking in modern society, the naturalistic fallacy: the prior that there's some inherent qualitative difference between the class of things occurring naturally and the technological/synthetic.
It's perfectly legal to dye food yellow with turmeric.
Many synthetic dyes are illegal for food, dyes which (unlike turmeric) have never been shown to have caused actual harm. But which (like turmeric) are known to be toxic at some dose not used in any food product, some 10x or 100x or whatever high multiple.
Curcumin is known to be pharmacologically active; has lots of (poorly understood) biochemical effects, hits lots of targets. It's never been demonstrated to be medically useful. The unjustified assumption—the magical thinking—is that, being a natural plant substance, maybe some of these random effects, randomly twiddling with the biochemistry of a human body without understanding what you're doing, might be beneficial—so people deliberately ingest it. That's accepted. It's the opposite with synthetic chemicals: if one's biochemically active in some poorly-understood way, you assume that mystery activity is toxic—not beneficial.
(Tangentially, you should probably avoid turmeric regardless of any of this, because the modern supply chain is contaminated with heavy metals, which cause poisonings[0]).
[0] https://www.consumerreports.org/health/food-safety/your-herb... ("Your Herbs and Spices Might Contain Arsenic, Cadmium, and Lead")