Hi Anh,
I am having the same issue when doing this as well. When using the building and trying to drag and next fields under tabs, this causes a Javascript error in Google Chrome and Firefox. It also causes all of the fields to be removed.
Only having one tab and simply listing the fields below, will make it appear on that tab, but if you add multiple tabs, it will not display the fields on any tabs by doing it that way.
I am unsure if this is an issue with tabs or with the builder.