Nested HTML spans are wrongly converted to XDOM

Steps to reproduce

Convert the following HTML to XDOM using html/4.01 syntax:

<span itemprop="credit">
   <span itemprop="author">Lorem Ipsum</span>
   <span itemprop="provider">Id sec amor</span>
</span>

Expected result

The produced XDOM should consist of a GroupBlock with attribute itemprop="credit" and containing two FormatBlocks.

Actual result

The produced XDOM consists of a ParagraphBlock without any attribute containing two FormatBlocks: the top level span with its itemprop attribute has disappeared from the structure.