Thanks, I will investigate this. I suspect it’s because the OP contains no text at all.
We use the OP text as part of the question in the QAPage schema (for topics enabled for Discourse Solved). If there is no text in the OP then we may need to either skip it or use some placeholder (maybe using the topic title in the text value would work) to prevent Google from showing this error.
We already skip individual posts now if they have no text, but it looks like we can have “questions” in this way without body text so will need to account for that.