Hi Miguel,
The mentioned problem occurs when we are using the EJ2 controls along with Report Viewer then we need to use the compatibilty script refernece to resolve this issue at your end. Please find the below help documentation for using the compatibiliy css for your reference.
link.https://help.boldreports.com/embedded-reporting-tools/faq/bold-reports-with-ej2-controls/
Regards,
Manoranjan R