parinfer-rust: Undo not working properly in Vim8 and Neovim

I just installed this plugin; I’m using Vim 8.0 on macOS with patches: 1-1650 included. I’m seeing some odd behaviour when I undo. Let’s say I have the following text in the buffer:

(defn f
  [x]
  x)

(defn g
  [y]
  y)

I go to line 4, press O followed by Enter and insert another function so that my buffer looks like this:

(defn f
  [x]
  x)

(defn h
  [z]
  z)

(defn g
  [y]
  y)

Now when I press u to undo, I get:

(defn f
  [x]
  x
  []
  z)

(defn g
  [y]
  y)

whereas I would have expected something like the buffer in the first example above. Pressing u more times doesn’t help - it keeps going through what appears to be a very long changelist but with no change being made to the buffer.