Sorry, I’m not able to reproduce this on server v5.12.3 app version 1.21.0 build 206
However, verifying that there definitely seems to be a “connection” issue when logging in to a v5.12.3 server as after trying to reproduce your issue here, and going back to the app to try to reproduce this current issue (I did not log out and back in, just opened the lock screen on my device), I got the grey notification bar “Connecting…” and trying to post a message results in the “i” to the right of the post.
The device does connect in about 4 minutes and then gives the notification “Refreshing messages failed. Pull up to try again”. If I then tap the “i” to the right of the post and then “try again” the message posts successfully.
I’m thinking that this current issue could be related to your issue here where the device doesn’t connect once you’ve logged in (but doesn’t give any notification either that you’re not connected)