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.
The merge fails silently (there are no errors either in the console or in the logs) and the posts are soft-deleted but not merged.
As @pfaffman pointed out, the problem is that such long posts, due to the max post length site setting (set by default to 32.000 characters), cannot be merged and the error should be displayed indicating that the merge cannot be done because the number of characters is higher than those allowed.