Re: We already know.
"Certain flavorings contain diacetyl and acetyl propionyl which give a buttery taste. Diacetyl and acetyl propionyl are associated with bronchiolitis obliterans. A 2015 review recommended for specific regulation of diacetyl and acetyl propionyl in e-liquid, which are safe when ingested but have been associated with respiratory harm when inhaled."
Sorry, the studies havent stood up to scrutiny. Besides anything else, the levels are 100-750 times less than combusted tobacco, and no one ever got popcorn lung from smoking.
Some flavours like custard did contain small amounts of diacetyl, they have since been reformulated to remove all but trace amounts. As part of MHRA testing, eliquids must be free of diacetyl and aldehydes before being allowed to be sold.