remove unwanted plugins it will work if same problem happens reply back
It could be plugin interference as Anto suggests, which can be also due to wanted plugins as much as unwanted. As a test, deactivate all, then activate one at a time until the problem recurs. The last activated is at fault. If the same problem happens with all plugins deactivated, it’s either your theme or the widget does not apply the “widget_text” filter. Also try switching to twentyseventeen theme with all plugins deactivated. If your filter is still not applied, the widget is not applying the filter at all.
What widget are your trying use? Obviously if it is a plugin or theme widget, that module needs to remain active for testing.