There seems to be an issue with both transparent glass hack (known), and now base default shader opacity with volumetrics.
Actually it seems to be Photoshop encoding in this trial of 2021. PNG and TIFF are both creating this result, it's not the volumetrics. I don't understand what it's doing to the files.
Does anyone know or have encountered this? If I make TG use alpha from colour for opacity it will work, but not just straight as an opacity map like we do for plants and everything like that.
Edit: OH YEAH! That's right! The Terragen plane is just weird. You have to use a card.
Interesting to note that semi-transparency is almost working in Path Tracer, though not Standard. Only issue is the volumetric, depending on how dense it is, makes the semi-transparency glow similar to subsurface scattering.