Thank you for the provided demo project. I have already replied you in the support thread you opened regarding this report. I will paste my answer here as well, but I will kindly ask you to continue any further communication regarding this topic in it.

Thanks in advance for understanding.

"The reported behavior would be the expected one and is due to the column virtualization of the control. Initially, when there are columns that are not visible in the viewport, tthe control will measure them on demand when the user scrolls horizontally. This behavior is reproduced when the width of the columns is not known preliminary. What you can do to avoid this behavior is to set a fixed value for the Width property of the columns, or set the ColumnWidth property of the control which will apply the same width for all columns."

All the best,
Stefan