hook_form_alter diseapers from module_implements cache

Problem/Motivation

We're using the GenPass Module which implements hook_form_alter().

After a variable time, the module seems to diseaper from the module_implements cache.
The module is still activated in the database, we can var_dump stuff from the module file, but the hook_form_alter is not called anymore.
We looked in the cache table (after changing it from BLOB to TEXT) and saw that genpass was absent from the module_implements cache.

Is there any reason why the cache would suddendly remove the module from the module_implements list ?