Hi Cristian,
Thank you for using Syncfusion products.
We tried to reproduce the issue at our end in IE 9, 10, 11. But we were unable to reproduce the reported issue. We have attached a sample for rendering treeview inside dropdownlist.
Please check the attached sample. If the issue persists, kindly provide us an issue reproducible sample along with replication procedure. It will help us isolate the root cause and provide you solution.
Regards,
Prince