Hi Yacinus,
I apologize for the delay in responding.
I am afraid that I am able to reproduce the issue specified by you.
Attached is the sample in which you can use the 'FormEditMode' for editing and the selection type has been set to 'Multextended'.
http://websamples.syncfusion.com/samples/Grid.Web/5.1.0.51/Forum-64541/main.htmWhen more than one records are selected and when you enter into the 'Editing' mode, the editing works for the last row which you have selected.
Could you please reproduce the issue in the above attached sample?
Thank you for your patience.
Regards,
Valli