How to create flexslider slideshow as a background in drupal 8?
Flexslider is a javascript library that makes a bunch of changes to the markup once it is loaded.  Instead of trying to put the images in as background images I suggest using position: absolute on either the flexslider itself, or the items you want to sit on top of it.

You may also need a z-index on some items to bring the to the front or send them to the back.