This might be intentional (though I really hope it isn't), but I figured I'd post it just in case.
Rainforest trees placed on the craftable Dense Turf cannot be "infected" by nearby Cocooned Trees, because spider_monkey_herd.lua line 52 only counts the natural-spawning deep rainforest turf as valid. This makes it impossible to have Cocooned Trees outside of the deep rainforest, for those of us who wanna use them to make our base pretty.
Steps to Reproduce
spider_monkey_herd.lua > line 52
This issue has been fixed in either a future version or updated in a previous version.