XI should be a moot in this discussion.
try using the User Interface (UI) to create your group summary.
You already have a group (just one?) using the OACUOR field.
click on the Sigma button (Blue E)
Insert Summary box opened up
select MITTRA.MTTRDT as the field to summarize
select Maximum as the calculate this summary option
select group #1 OOHEAD.OACUOR as your sumamry location
select OK
this should drop the max into your group footer.
does this work and display the correct value for your group?