When I imported a very simple Ppro projekt into After Effect Beta 220.127.116.11 (Build 79) or After Effects 18.4.1 (Build 4) the PNG file lacks alpha channel. I can not change this in File->Interpret Footage->Main.
But if I choose File->Replace Footage->File and select the same PNG file it imports/replaces with alpha channel.
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And the same goes for the AI file I noticed. No alpha when importing premiere project.
Fine after replace file with same.