The two cases of popcorn lung were directly linked to the binding agent propylene glycol in one batch of “overseas” vape liquid.

Propylene glycol is a bog standard diluent for e-liquid, usually used in combination with vegetable glycerin. I don’t know which country you’re in but at least some of its domestic e-juice makers use PG for sure.

Popcorn lung is associated with diacetyl and other diketones used in food flavouring (it’s called popcorn lung because the harms were first noticed in workers making popcorn). It should be banned for use in vapes but Big Food is still fighting off compensation claims so the various regulatory authorities worldwide have been slow to name it as a risk of vaping. Some e-juice manufacturers take pains to avoid it anyway.