From my experience, this report isn't saying that the liquid contains 0.4% diacetyl. 0.4% is their limit for testing, i.e. they can't detect anything under that limit. The actual amount might be 0.39% or 0.2% or 0% - the company doing the testing can't tell.
I'm not trying to deny your throat irritation or anything like that. Just trying to clarify a bit.
From speaking with Tom, the Owner of Capella's, what's showing up isn't Diacetyl, rather, the Acetyl Proprionyl, which is in the mix and can show up as Diacetyl as it's almost identical with very minor differences in the chemical structure. It's not Diacetyl, it's one of the alternatives, but in testing, it's going to show up unless the GC/MS testing can dive in deeper than just looking for that one chemical. I don't even know if GC/MS testing would be best suited given the similar nature of the chemicals, but I'm not a scientist.