Can you make sure that all your function terminate and are joined
(Thread.join). I think this will help to make sure that the thread exit
and call thread_kill (see OCaml source code).

If you take a look at thread_kill there is a function stat_free + set to
NULL things called stack_low, stack_high... Maybe all the data you are
seeing come from this...

I am not sure that Thread.join will free anything, but it will help you
to be sure that your thread has exited correctly.

Regards,
Sylvain Le Gall