As per the title, in a world generated with seed 5018746510160316681 (the same one where I discovered #20) in a Mushroom Fields biome at 44, ~, 5844, I found a handful of large mushrooms that had generated with the new tall mushrooms growing out of them. I figured that this is unintended, so here I am to report it. This can occur with brown and red tall mushrooms, and I saw them on brown and red large mushrooms and green large glowshrooms.

Four instances of this can be seen in this screenshot.

This is intended.