I will clear my chat history with someone and all the messages will disappear just fine; however, if they whisper me again, the ENTIRE history comes back.
Blizzard support suggested this:
- Press the Windows Key + R and type:%PROGRAMDATA%. Delete any Battlenet and Blizzard folders you see in this location.
I did this, restarted battlenet, and the issue still persists. The next suggestion from the support agent was to post the issue in here so you can investigate it.