It was some time ago. I saw it mentioned on several threads when we used to see more frequent problems with DNS servers that rate limited inbound queries.
Here is one example that explicitly says LE starts at the deepest level and works up. Although it pre-dates your post from Aaron so maybe was changed: "networking error looking up CAA for de" - #2 by mcpherrinm Update: Also see Aaron's comment below which seems to contradict this