The term popcorn lung came from the 70s or 80s where they made the butter flavoring for popcorn, and most the people affected had been working in very high concentrations of the mist of the butter flavoring, thus the term popcorn lung. Is the risk high with vaping on buttery custards and such liquids that require Diacetyl and AP to get their taste right, it is hard to say what risk factor in a percentage is there due to many variables, wattage, how much vapor you are inhaling each hit, length of usage, but the possibility is there. Are you in any immediate danger within the next few months, probably not, 6 years down the road, who can say, we don't have the data, so all we can do is arise caution given the history of what Diacetyl and AP have done and been documented thus far. We don't know what the threshold level of exposure is, or how to quantify it down to using this many (some number) milliliters of juice in this (some time frame or vaping level (sub-ohm, super-sub-ohm, median, or standard vaping level), etc, etc, etc.
Simplest solution, I can advise, is find alternatives that are similar to what you normally vape that do not possess Diacetyl or AP, decrease your usage and find other alternative flavors, or continue using as you have been. As each vaper is unique and their tastes are just as unique, without much more info of flavor profiles or exact liquids I can't suggest alternative vendors or flavors, but above is the gist of it all as far as I know. Is it something you should be stressing over enough to cause anxiety, IMHO, not really as with the cases of above that had been after 10+ years concentrated exposure that caused it, literally working in a mist filled environment 8-12hrs a day for years before the cases started showing up, your exposure level is every hit and few minutes it takes the vapor to disipate, not constant exposure. I have narrowed my flavor arsenal down considerably to just a few change up flavors, mostly fruits that I rotate to from just straight unflavored juice I ADV all day long, due to we still do not have the data to quantify what flavorings and such cause risk and at what concentration levels.
hth