Hi Son,
Sorry for the delay in responding.
I tried to reproduce the issue with the attached sample with multiple forms. I am afraid that I am not able to reproduce the issue. Could you please modify the sample so that the issue could be reproduced? This will help us to resolve the issue soon.
Thanks for your interest in Syncfusion products.
http://websamples.syncfusion.com/samples/tools.Windows/F58450/main.htmBest regards,
Golda