> I don't think the effects were expected, at least not officially.
I'm not sure about that, and if they weren't this was gross misconduct either way as issues related to dioxin contamination were either known or strongly hinted at before the operation started.
> This was an herbicide, not a chemical weapon.
The herbicide itself (actually a combination of two herbicides, 2,4-D and 2,4,5-T) is relatively innocuous, but during manufacturing 2,4,5-T can be contaminated with 2,3,7,8-tetrachlorodibenzodioxin — the colloquial "dioxin" (by accidental overheating which is a significant risk of high-temperature 2,4,5-T production).
This contamination was revealed in 1952 (by Monsanto) and the toxicity of TCDD was published/revealed as early as 1957 (and Boehringer who manufactured 2,4,5-T using a low-temperature process warned other producers of the high-temperature process's risk and provided mitigation suggestions).
Operation Ranch Hand started in 1962.