Path filtered history not maximally simplified

While investigating a path-filtered history in gitk, I noticed a layout like this:

     --A---------M--4--N
        \       /     /
         1--2--3-----5

There are no commits displayed on the upper line between A and M. Shouldn't this have been simplified to:

                  4--N
                 /  /
     --A--1--2--3--5

The gitk command used is simply:

     $ gitk some/file

and if I run

     $ git log some/file

then M is reported with two parents (so it's not gitk's fault). The above is just the most recent part of the history; there's a lot more older history, and it is non-trivial with numerous merges (in the complete history). In particular, there _are_ commits on the upper branch between A and M, but none of them touches some/file.

Do I need to adjust my expectations or does this look like a bug?

-- Hannes

PS: This is with 1.5.5.rc0.21.g740fd.