In the latest version of Kendo UI, various input controls like the NumericTextBox or the ColorPicker now consist of an input field and a button control (used for increasing/decreasing the value or opening the dropdown).
The border-radius for those widgets can be controlled by setting the "rounded" option. At the same time, when using SASS themes, a button's default border-radius might be set using the $kendo-button-border-radius variable. The buttons inside e.g. a NumericTextBox do not override the theme's border-radius which leads to an outcome like this:
Widgets that use Buttons internally (and therefore offer no way of overriding the button's border-radius by setting its "rounded" option manually) should override the default styles.
Unfortunately, I could not reproduce the behavior in a DOJO, because I cannot transpile SASS themes there.