Mail grouping/thread bugs

After updating to 8.0.2703 there seems to be something weird happening to how the client handles email threading/grouping.
When I receive an email and reply to it, EM Client doesn’t always groups the reply together with the original message, but sends the reply as a new separate email. So basically many of my mails now have two individual threads. One with the original mail and one with the answers. If I receive an reply on my sent email and answer it, it seems to thread them together

Anyone noticing the same issue or has a solution how to fix this?