Later on, I got a notification on my phone about having received an answer. It was a good one! When I tried to upvote the answer using the Android app, though, I wasn't allowed to: it said I hadn't joined the site, apparently not having caught up with the fact I had done so a couple of hours beforehand.
I apologise for the low battery indicator in these screenshots. :) I plugged it in to charge minutes later, I promise!
When I said I wanted to join, though, I couldn't, unsurprisingly, on account of already having done so:
I closed the app entirely and reopened it, and then the app was happy that I was a member of Anime & Manga. But it wasn't updated whilst the app was running.
The app should probably get informed of this. (Though this may be low priority due to not occurring all that often.)
This occurred on the Android App v1.0.51 on a HTC One running Android 4.4.3.