Same issue for me. Can't find a way to do this. Apollo had a message at the bottom of the screen that said "view all comments", essentially. So far, Lemmy doesn't seem to have an equivalent anywhere.
I was looking for the same as well. For now I am bookmarking the discussions when replying to them but it would be much easier and more convenient to be able to access the complete thread from the inbox.
There is similar report above, and dev said that solution is in the works, and should be available soon.