Hi Øyvind Knappskog Olsen ,
We have reviewed your query, and when attempting to merge cells, the grid does not allow cell merging if the SelectionMode is set to anything other than None. To overcome this limitation, you can handle this behavior by setting ExternalExceptionThrownEventArgs.Handled to true using the ExternalExceptionThrown event. This will allow to enable the merging with selection and edit merged cells.
For further reference, please refer to the User Guide.
Regards,
Sweatha.B