Hello!
Is it possible to avoid alpha propagation as shown here 
while still using groups?
snapshots don’t work for me, the groups are too big, like 20000*20000 big in some cases. May be there is another way?
Can you give us more information? What exactly are you trying to achieve?
I don’t think that is possible unfortunately, but I would love if someone has a simple solution for this. 
It looks really terrible when using alpha transitions and objects shine through each other.
Just remembered I wrote a blog post on a similar topic a couple of years ago, about how to create a proper fade in/out effect on scene transitions. It’s not exactly the same thing that you’re looking for, but maybe it could be of interest anyway.
http://bitstopixels.blogspot.com/2018/06/corona-reloading-scene-with-transition.html
You may have missed one part from the documentation:
Bounds
A display group’s width and height are determined by its children, specifically the union of the children bounds. In contrast, snapshots have a pre-defined width and height, so children which reside outside those bounds will not render.
https://docs.coronalabs.com/guide/graphics/snapshot.html#bounds
It doesn’t really matter if you have massive groups as long as you keep your snapshot size small (and there’s really no reason to go past the screen bounds).