In the UncontrolledTabs component, default props are merged using object spreading:

const { className, ...attributes } = { ...defaultProps, ...props, };

Even the defaultProps.className is defined ('react-tabs') and props.className is explicitly undefined, the spread operation will overwrite the default and set className to undefined.

As a result, the default class name is lost. It's unclear whether this behavior is intended, but it can lead to unexpected issues, especially when upgrading from older versions where the default class was always applied. Projects relying on the default 'react-tabs' class for styling or selectors may break silently.