Huh. I wonder if it trusts 32-bit math (skimming the code, it looks like maybe). I found out trying to implement chacha20 (which I might get back to some day) that 32-bit integers sort of work but act strangely because javascript. I think the Basics module warns about this.
The roll-your-own approach of the icidasset packages seems probably less efficient but it’s guaranteed to work.