I've started looking at the details of the Headless Services GraphQL endpoints in Sitecore recently. And as part of this research, I got a bit confused trying to test queries in the Sitecore UI. I've worked out what was up, but maybe others will find themselves in a similar situation, so:
I came across an interesting issue generating a Sitecore Package recently. Googling the issue didn't give me the whole answer, so it's time to enhance the internet again with a explanation of what I saw and why I think it happened...