In an old world (both in-game and real-life old), after learning all the recipes, I recently started to get blueprints from tumbleweeds that I already know.
Much earlier I got a lot of "unknown" blueprints, but these disappeared once, I believe an update fixed that bug. For a longer time I didn't get any blueprints from tumbleweeds. In the last few weeks however, there are blueprints again, and not "unknown" but ones with known recipes.
I don't know if this happens in newer worlds too, I haven't learned all the recipes in any other world yet.
I'm not exactely sure if it's a bug or not, but it looks so to me so I posted it here.
Not sure... have a really old world and know all the prototypable recipes. Then open tumbleweeds.