Z depth is definingly needed when using alpha and grease pencil or at least that’s what I’ve been told.
You may need to save as openEXR. Blender’s renders are 32bit float. PNG is straight 32bit. So, some information gets lost. Also you are on a Mac which has a higher range of colors on most of their displays which means you can see more subtle changes than most PCs. If I remember correctly PNGs are straight alpha instead of premultiplied alpha which means emissive alpha often times get lost. I’ll have to check my notes. I think I have some color management settings that could help. Just incase openEXR isn’t options for your needs.
