While collecting lingonberries, I had been automatically hitting my collect sprout hotkey while harvesting from habit, and to my surprise my inventory had a couple.
Presumably they still have the sprouting age category and just don't display?
The sprout cannot be planted on tundra, but if you plant it on a grass tile it instantly turns it into tundra.
Edit: I've gotten sprouts from age categories mature, old, and very old so far.