Are you using quick reply? Seems like something something similar happened to me and I decided to not ever use quick reply again. You lose the email addresses that were in the conversation. Try both ways of replying and see if quick reply is the problem for you.
The other problem I have is putting email addresses in an email without first adding them to my address book. Maybe I don't need them in my address book. In the old system, I could highlight them and add a long list. I finally got the format of a list of people's emails in my list to send, and then it told me it couldn't send them because I had too many. So I just did it on gmail.
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