Is this a bug in overlay property list handling?

[Lennart Borgman (gmail)]

I am not sure if I have hit a bug or if I am doing something wrong. I do 
something like this in my code:

  (setq syntax-ppss-last-min
        (copy-tree
         (cons (1- last-pos)
               (if is-main-mode-chunk
                   (mumamo-with-major-mode-fontification main-major
                     `(parse-partial-sexp (point-min) last-pos))
                 (parse-partial-sexp 1 1)))
         ))
  (overlay-put chunk-at-pos 'syntax-ppss-last-min syntax-ppss-last-min)

If I test this immediately after with `overlay-get' the value looks ok.

However, later when I read these values with `overlay-get' it seems like 
the property list has got corrupted. The car of the value stored in 
property 'syntax-ppss-last-min is nil.

I use copy-tree to try to avoid surprices like this one. Maybe I should 
do something different? Maybe this is a bug in overlay property list 
handling?