ColumnGroups makes the ordinary column headers disappear

[Robert]

I have  a Telerik gridview, with visible columns, works fine.  I now want to add column groups as in the example on

https://docs.telerik.com/devtools/wpf/controls/radgridview/columns/column-groups

 

But when I added columngroups-tag and some column groups, it results in a grid with only the column groups as headers and

the real column headers are gone. See attachment for my markup

[Robert]

btw. Just ignore the swiggly lines at "MyGridStylesSelector", I did some renaming before the screen shot, replacing texts with "My*", to protect the innocents 

[Robert]

SOLVED:

Turned out to be HeaderRowStyle that had a fixed height.