After two days of searching and debugging I decided to give up and ask help here...
I'm building a multistep form in Drupal 7 with 4 steps. The user_register_form is in the second step. For building this multistep form I used the help of http://www.grasmash.com/article/building-multistep-registration-form-drupal-7. I also did some modifications so other modules that alters the user_register_form would be called. I used the modification from comment http://www.grasmash.com/comment/127#comment-127.
My problem: After submitting the second step, the user_register_submit() function is being called. Although it should move all submit handlers to the $form['next']['#submit'] and call all submit handlers after the last step.
I tried to unravel the whole form handling within Drupal and explore every single function used. I found out that somehow the #submit handler ends up in 'triggering_element', which causes the form to fire the submit handlers directly after step 2 instead of 4 ... but can't figure out how and when this element is set.
Any ideas where to look, or how to investigate this more or better?
Edit: After better reading I came to the conclusion that the way the multi step from grasmash works, is aimed to execute the submit handlers after every step. Although I thought that the submit handlers were moved to $form['next'] so they could be executed in the end. So now I'm wondering how to change the code so all submit handlers will be executed after the last step?