I can confirm the issue, thank you.
The fix will be included in the nearest release.
Also, please note that starting from 6.3 buttons accept several predefined classes instead of types such as “form” and “danger”. These types will be deprecated in the future major updates.
In case of custom styling via css:"some_class", the selector has to include webix_button with the following structure: https://snippet.webix.com/oxkrzdaw