I doubt that it's a bug, it's pretty basic stuff.
I'd be more inclined to think that @percent may be returning a null in it, which would appear as blank, but if Lisa has experienced the same thing, you may be right.
Have you considered creating your own running total using the 3 formulas method?
Group header formula:
global numbervar MySum:= 0;
Details formula
global numbervar MySum;
If {Employee.b2_dpremiu}>{Employee.b2_epremiu}Then
MySum :=MySum+{Employee.b2_dpremiu}
Else
MySum :=MySum+{Employee.b2_epremiu}) * {@Percent}
Group footer formula:
global numbervar MySum;
MySum
-k