Does this issue only happen to the two users?
Is the conversation between A and B hidden every time?
Do the two users try in other machine and what is the result?
Could other users’ private chat history with one of the two users show properly?
From desktop client side, clear the client cache by deleting the content in %appdata%\Microsoft\Teams folder and update client to the latest version.
From web client, sign in Teams client in InPrivate mode and observe the result.
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