I've got version 1.0.24 of the android app.
On my computer (the one I'm posting from now, as a matter of fact) I
- signed up for the Chemistry SE and posted a question.
- I got an answer, which notified my phone app.
- After viewing the answer on my phone I decided I wanted to post a comment
- When I clicked "Add Comment" it popped up "Add to your sites?" dialog asking if I'd like to join.
- When I click "Join Site" I get a dropdown notification, "You're already a member of Chemistry."
- Cancel simply closes the dialog.
- I tried using the back button to exit out of the app and came back in - with exactly the same behavior.
I'm about to try flushing my stored data for the app.
Flushing my stored data worked.