Fun fact, apps can't wake from APNS if the user killed the app (swipe up) last time instead of switching away normally. Apple publicly said something contrary to this at one point, so it might be surprising that Signal can work this way. The notification itself will still come through outside the app, so I wonder what you see, probably some placeholder text?

Yes there's a placeholder in that scenario Something like 'new message available' if I remember correctly (I'm no longer on iPhone, so can't verify)