Deleted conversation history persists in the Skype for Business on Mac client

Considering the following scenario:

• User A and user B are using the Microsoft Skype for Business on Mac client.
• User A sends an instant message (IM), for example, "test1" to user B, and user B does not respond.
• After several minutes, user A sends another message, for example, "test2" to user B, and user B still does not respond.
• User A sees a conversation with user B under the Chats tab in the Skype for Business on Mac client. The conversation shows messages "test1" and "test2."
• User A deletes the conversation with user B.

After some time, the conversation with user B appears again in the Chats tab with only the message "test1." It's expected that all messages in the conversation with user B will be deleted.

To fix this issue, install the following updates:

• The July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Web Components Server.
• The July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Front End Server and Edge Server.