This definitely appears to be a change in behavior between 12 and 13. I think I know how to solve this. Go to layout mode for the short report. Double click on the subsummary layout part to bring up the Parts Definition dialog. Click OK. In the next Dialog box select Print Above instead of Print Below. Not sure why you have to do this in MFGO 13 but it worked when I tried it.
I guess that was about the only thing, I did not try - but it worked.
You are King - and I think FileMaker has a bug here.