Noticed the difference between rP4c143ad3b2c4bb6e9d15e2eefda5432455fc3a9a and rPccd4ae56382762fa2631e1c0a521e4ebaa5f2926.
The first one appears on T4205 because it contains "Ref T4205" in the summary. The latter one does not appear, even though it has "Maniphest Tasks: T4205" in it (the first one has this as well).
Generally I think commits should be picked up by the task if the diff they reference is associated so that there isn't inconsistent behaviour (it also means that all diffs that are associated with a task that get closed off get the nice "D1234 / rP4c143ad3" format).