I have a report I started creating in 3.7-RC1. I have opened it in 3.8 without saving and I have a copy that was modified in 3.8.
I have the Report Header sheetname property set to "Summary" then I have a page-break-before = true on the group header and have the group header "sheetname" property set with a style expression (="Const value " & [my_group_field]).
In 3.7-RC1 and 3.7 GA this way of setting the worksheet name with a style-expression works fine. I can open up this report and run it in 3.7 and it breaks where I want and each of the sheets are named with the expression using the value of my group field. Even the report modified in 3.8 opens fine in 3.7 and the style expression still works.
In 3.8 the first group sheet after the page break is called "Sheet 1" while the second sheet uses my expression.
Since the group header is empty I have tried making it visible and invisible with no effect. I have also tried putting these two properties (page break and sheetname) at the Group level instead of the group header - again no effect.