Date Repeat Field validate function not being called on node save -> No repeating dates being stored

We've had a strange issue where when you save a node with a Repeating Date field, the date_repeat_field_widget_validate function doesn't get called and that means that the repeating dates are never saved into the database.

Here's an example image of what is shown in the node after it's saved: only a single date is saved and no repeating dates are shown.

In date/date_repeat_field/date_repeat_field.module there's a piece of code in date_repeat_field_field_widget_form_alter that adds the function date_repeat_field_widget_validate to the form element's $element['#element_validate'] and that's all good, the function gets added to the form validate array but when you submit the form, the function doesn't get called in the end and no repeating dates are saved into the database.

The website has been developed with Drupal Commons.

My idea is that somewhere in the code at one point or another the $element['#element_validate'] variable gets overriden but I haven't been able to find out where.

Has anyone had a similar situation?

Thanks for any help!