Hi @svenni
Thanks for the debugging info.
I think the wp-pagenavi plugin is hooking into the the_content filter and modifying the value based on the comments.
In fact, I’m 99% sure it is.
All you need to do is run the the_content filter on the ACF value, and your output will render correctly!
echo apply_filters('the_content', get_field("pagenavi-site"));