The document processing section in ODF 1.2 CD02 REV01 says:
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If a foreign element has a <text:h> or <text> ancestor element and is a child element of an element which may include <text/>, then the element's content may be interpreted by conforming OpenDocument processors, and the document itself shall be valid against the OpenDocument schema if the unknown element is replaced with its content only.
What is missing is a requirement that also the ancestor elements of the foreign element up to <text:h> or <text> must include text.
Reason is that the processing of foreign elements described here shall apply to all those elements that constitute the paragraph's text. That's for instance the <text> element and its <text:span> child elements. But the rules shall not apply to elements that may appear in a paragraph, but do not contain its text content, like <text:frame>. Without the additional requirement, they would be applied to for instance a <svg:desc> within a <text:frame>, because <svg:desc> allows <text/> and has a <text> ancestor>. With the additional requirement, the special processing of foreign element stops at <text:frame>, because this element does not allow <text/>.