Here is a problem I have never encountered before. I had closed a ticket for a customer  and she could see the resolution in the history but then the history disappeared and it had disappeared when I went into the ticket as well. I re-opened the ticket and eventually for me the history re-appeared so I was able to email the details to the customer as she could still not see it.

It is really weird as it comes and goes as in this snip. I am using Internet Explorer 11 and have cleared the cache