Shortcodes inside [nextpage] are not working with "Previous Next" Buttons

Hi guys,

first of all: thanks for this amazing plugin 🙂

I've a strange issue with the Previous & Next Buttons: no shortcodes are working inside [nextpage].
If I choose another button type everything is working fine, but not with Prev/Next.
Take a look at this example live page: http://musikerplus.de/gitarre-bass/steve-stevens-der-klangzauberer/
I put a formular shortcode inside the text, live I've said, it's working with all other button types,
I just can't explain that to myself...

In the past I've had also issues with the wordpress wpautop function:
I fixed it with this piece of code in my functions.php file, maybe the error is related to that....

remove_filter( 'the_content', 'wpautop' );
add_filter( 'the_content', 'wpautop' , 12);

But only with prev/next buttons, all other button types are working well...

Would love to hear from you, many thanks in advance,
Wellington