Hello, Suren.
Try to implement notes:
Note 1764696 - After warm transfer of calls, calls appear in new window
Note 1774919 - Warm transfer entails a multisession
I know that they are about calls and you have the issue with emails. But the correction provided in these notes fixes the general multisession manager.
Hope this will help you.
PS: It's better to open your own discussion in case you have any questions. Not to mix different questions in one discussion.