Consider the following scenario:
You select the Reopen my conversations when I sign in option in Options > General in Microsoft Skype for Business 2015 (Lync 2013).
You're active in one chat room that's not in the contact list. At the same time, there are some tabbed conversations that have some messages coming in from other participants.
You sign out of the Skype for Business client without manually closing the chat room.
In this situation, when you sign in the client again, the chat room is reopened as expected. But you may find that the total number of the messages of the chat room is indicated instead of that of the unread messages.
To fix this issue, install the July 10, 2018, security update for Skype for Business 2015 (Lync 2013) (KB4022225).
Note Lync 2013 was upgraded to Skype for Business in April 2015.