When I use "the_field" with a wysiwyg field, it displays the full content of the post. I found a similar, but unanswered problem in the plugin forum here
I also tracked it down to the filter function. My best guess was a filter applied to "the_content" and after searching all theme files and installed plugins, I disabled the Advanced Fancybox Plugin and now it works.
So if you run into the same problem, search for something like "add_filter('the_content', "...
Hope this helps someone.