Thank you for contacting Syncfusion Support.
We have implemented our SfTextBoxExt based on MS TextBox behavior. While checking the focused behavior in MS TextBox, it also has same focus behavior as like our control.
We have prepared the workaround sample to meet your requirement. In this sample, we have overridden the SfTextBoxExt style and avoid the changing the Background color to White when the Visual State is "Focused".
Please download the sample from the below link.