Hi Florian ,
It seems you have encountered a bug in RadGridView. May you please share the code relevant to restoring the column groups . I need to be able to reproduce it here and I will take care for the proper fix to be provided.
Meanwhile as a workaround you may use a small hack to perform a "dummy" action to trigger rearrangement of the common headers e.g. add a fake column group to the column groups collection and remove it . A proper moment to do this would be after data has been loaded.
Again any hints , code snippets etc. that may help us reproduce the issue in a local project would be highly appreciated.
the Telerik team