Yes, I can repro it.
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.