SVN lift merges: No second parent / No Merge: info

I am trying to lift an SVN Repository using the current master of reposurgeon. In the resulting git repo some merges into a branch are missing the second parent and have no "Merge:" line in the log. They are not considered a merge, which makes tracking hard.

I could not find a common criteria what is considered a merge and what not. The svn:mergeinfo looks plausible. That is: mergeinfo is changes on SVN root and one or more of the direct subdirectories under root.

First question would be: what could be the cause for reposurgeon to consider a checkin a merge and what could prevent this?

Addition: It looks like the merge is not considered a merge if the original commit adds a file.