Answer by Glade Mellor for Bootstrap modal appearing under background

I simply removed the position: fixed style from the _modal.scss file and it seems to work fine for me. I still have the background and it works as expected.

// Modal background
.modal-backdrop {
  /* commented out position: fixed - bug fix */
  //position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  z-index: $zindex-modal-backdrop;
  background-color: $modal-backdrop-bg;
  // Fade for backdrop
  &.fade {
    opacity: 0;
  }

Kudos to Rick Strahl for the guidance: https://weblog.west-wind.com/posts/2016/Sep/14/Bootstrap-Modal-Dialog-showing-under-Modal-Background