hi-lock mode problem

[harven]

It seems that if a file contains more than 10000 characters,
and a hi-lock pattern is specified near the end of the file,
then this pattern is not recognized by hi-lock mode.

Let me give an example. Just put at the end of a file
  *containing more than 10000 characters
  *with hi-lock mode enabled (M-x hi-lock-mode)
the line
;;  hi-lock: (("h" . 'hi-blue-b))
then try to activate the pattern with C-x w i. Nothing happens.
If the hi-lock line is at the start of the file, it works,
all occurences of the letter h are highlighted.

I didn't find anything in the manual.
Is it a bug ? Or did I miss something ?