This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
Found while I was fixing up other content, that Behavior Tree assets are being marked dirty by just opening them into sub editor.
Result: BT asset opens in subeditor and is marked dirty with no edits made.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-34728 in the post.