Docstring folding seems broken in vim8 (but not nvim)

[kalekundert]

Here is a minimal example:

"""
Hello
"""
spam = "eggs"

In my hands, nvim correctly folds the three lines of the docstring and leaves the spam line unfolded. vim incorrectly opens a new fold on both """ lines. I'm not totally sure if this is due to some mysterious difference between nvim and vim, or if I somehow have the two programs configured differently.