Greetings from Syncfusion Support.
By default, SfNumericTextBox’s ValueChangedMode is OnLostFocus. In this mode, SfNumericTextBox’s Value updated while its focus gets lost. But, if we press button in command bar directly after entered the value in SfNumericTextBox, then its focus not get lost as per command bar behavior in UWP. So, you can resolve this issue by changing the ValueChangeMode of SfNumericTextBox as OnKeyFocus.
After analysis in source level that we concluded that we can achieve your requirement in sample level by setting “OnKeyFocus” value to this “ValueChangeMode” property.
We prepared the sample for your requirement. Please find the sample from the below link.