Hi Christophe ,
Thank you for your update.
It is hard to trace why this issue is arising without seeing it in a live application. We have searched our support database for any mention of this problem, and your incident was the only hit. This is a new one for us.
Is there any possible way for you to provide us with a sample that shows this issue? Can you reproduce it in our Printing sample/attached sample? It will be easier for us to have a closer look into the problem and to resolve it.
Please refer the below sample with MultiColumnComboBox that works fine without any issue.
http://websamples.syncfusion.com/samples/Diagram.Windows/F70230/main.htmWe appreciate your patience, and Thank you for the interest shown in syncfusion Product.
Kindly let us know if you have any questions regarding this issue. We will be glad to assist you.
Best regards,
Nagaraj