CI 3 + HMVC form_validation library problem

You have a double underscore in the validation rule function name:
[quote pid='4456' dateline='1422814140']

Code:

$this->form_validation->set_rules('password', 'New Password', 'callback__check_password');

[/quote]

Try using only one underscore and the function should have no leading underscore in its declaration

Your use of an underscore in the method name may be invoking the legacy code for a private function:
From the CI 3.0 docs:
CodeIgniter-3.0rc/user_guide/general/controllers.html#private-methods

Prefixing method names with an underscore will also prevent them from being called. This is a legacy feature that is left for backwards-compatibility.

CI 3.1 Kubuntu 19.04 Apache 5.x  Mysql 5.x PHP 5.x PHP 7.x
Remember: Obfuscation is a bad thing.
Clarity is desirable over Brevity every time.