I remember a discussion here on Meta.SO about Edit Summaries for multiple edits within the grace period, but I can't find it. At the time of that discussion, when one submitted another edit within 5 minutes and one submitted another Edit Summary, only the second summary was used. Now it seems that the summaries are concatenated. (If the 2nd summary is empty, this has the effect that something like
; added 32 characters in body ends up as the summary text. Before the change, nothing was added in this case.)
When did this change happen, and is it documented somewhere? I don't see it in the Recent feature changes to Stack Exchange.
UPDATE: It seems that I got it all wrong in my recollection of how those summaries work, see the answers.