Occasionally, messages will get "stuck" and will not send from the mobile app. When this happens, the resident can send multiple messages to Nearby chat which cannot be seen by others. When they are in this state, their IMs also do not display to those who they message.
After a period of 10-15 minutes, this condition of being "stuck" will abate, and all of their messages will display at the same time in Nearby and in IMs.
Restarting the app also fixes the problem. The messages that they sent previously never come through in that case, but any further messages that they send after restarting are fine.
I have witnessed this multiple times, once for myself while using the mobile app on Android, and multiple times for new mobile residents, which is very frustrating for them. I'm uncertain if they are on Android or iOS.
I would say that it's not a super common occurrence, but I witness it happening once or twice a day to someone, so it's not all that rare, either.