Drush pm-update: How can I stop it from trying to update more recent dev releases?
Basically, this sounds like a bug that should be addressed in the drush issue queue.  I'm guessing this is recent; it used to be that Drush would never go from dev to stable unless you used pm-download.  Now it is being a bit overzealous.

As a workaround, you can use the --lock option to stop Drush from updating the module at all (drush pm-update --lock=heartbeat), but this will also exclude newer dev releases as well.