Look for documentation about how precisely modules parts are loaded (Drupal 7)
After a lot of tests, I'm now pretty convinced that, despite it is not mentioned in the release notes page, a change happened in 7.34, which consists in that, now, the full-bootstrap condition is not reached yet when the MODULE.module file is loaded.

According to module_load_include() page, this function requires Drupal to be fully bootstrapped: this clearly relies to the use of drupal_get_path() (see the function code in this page), which is not available without full-bootstrap.

It is why my (global-scoped) module_load_include() calls are executed but without result (see 'file: mymodule.module' in my question above).

I had to modify my module, including a dedicated module_load_include() call inside each function which needed it, like this:

&lt;?php

function mymodule_some_hook() {
    module_load_include('inc', 'mymodule', 'mymodule.core');
    # ...
    myfunction();
    # ...
}


This way, now all works fine anew.

I verified it is the same with Drupal 7.36: my module doesn't work in its original implementation, then it does with this modification.

Hope this helps.