Why is Gajim showing a viewed message a second time?

Question

xampp

It costs Gajim 0.16.5 on both Windows and Linux machines.
Sometimes there is a situation when, after starting the program, it shows a message (blinks as unread), which has already been viewed by the user before.
Patterns are hard to follow. Sometimes it shows, sometimes it doesn't.
The phones have Conversation. Works perfect. So the problem is not with the server, but with the client.