You really should ask this in
http://discuss.grapheneos.org/
https://lemmy.ml/c/grapheneos.
It’s a very helpful community. Someone will either explain why or point you to a FAQ. I bet it has something to do with the built in reader being insecure.
Edited… I totally gave the wrong url. Sorry. Corrected.
Activity Manager
I’ve used it to launch certain parts of apps. Doesn’t always do what I want. And you have to sift through a lot of useless stuff. But it works.
https://github.com/sdex/ActivityManager
On F-Droid also