Okay, I think i'm on to something. It appears my issue isn't the underlying datatable or the binding.
In a post above Alexander wrote:
The RadGridView control uses UI virtualization - its row and cell elements are created only for its visible data rows and columns and then they are reused while scrolling, sorting, grouping and so.on. In your case, the custom group header cells are reused in other group rows while scrolling.
I have 10 groups with somewhere near 20 data rows per group. On the initial display I want the first group expanded and the rest collapsed. Some of the collapsed groups don't fit in the grid window and get clipped.
When I try to iterate the header row using
GridViewGroupRowInfo gi = rgvMain.Groups[i].GroupRow
bool test = (bool)gi.Tag;
The Tag values are null for the group header rows that are off screen/clipped. If I initially display all groups collapsed so that they're displayed on the screen, everything works fine.
Is there a trick to get around this?