In legacy ( using simplified xml ), it is possible to add custom tags inside <header> tags.

In richtext we do not support custom inside headers, so the conversion tool for ezxmltext to richtext should move such custom tags below the header

Edit:
Custom tags can be inline or block. Inline custom tags are supported inside <header>
By default the conversion script will treat unknown custom tags as inline and translate them using <eztemplateinline> tag.
This issue is therefore invalid