https://faq.soylent.com/hc/en-us/articles/203962069-Vitamin-...
https://faq.soylent.com/hc/en-us/articles/204547005-Magnesiu...
I'm not enough of a nutritionist to say whether that's really sufficient, but they do consult with nutritionists, so it seems likely they've at least thought about all this stuff. Proving there's something wrong with Soylent is going to take more research than anyone's likely to casually do for an Internet discussion.
Similar for the Magnesium, it was chosen for a neutral taste rather than based on bio-availability. Even though Magnesium Oxide is barely processed by the human body at all.
I'm not going to get into it because this stuff gets complicated really fast [1] and two non-scientists debating it on the Internet gets silly.
[1] https://ods.od.nih.gov/factsheets/Magnesium-HealthProfession...