So I've been having this issue for a while and also reported it into the version's discussion thread (but got no answer, since I'm posting this here again). Basically after I converted my map's materials to use PBR, the generated imposter textures are fully black (and the game also refuses to use the working textures I have included inside the mod), resulting in trees looking like this: I really do not want to convert back to Materials V1.0, as I spent a lot of time converting them in the first place. About the opacity map issue, they simply do not affect shadows (and also are completely ignored by the imposter generator):
