I had a user that posted a single post with some text, then two very long posts that contain logs, and finally another post asking for help. I tried to merge these four posts. After I selected all four posts and clicked merge, only the last post of the selection remained without merging any other post to the last one. Others were marked as removed. I restored the posts and tried again but got the same result.
I have 2.7.0.beta5 (5c84f702b0) installed.