Custom class names not rendering outside of views preview

I'm having a recent issue where after updating drupal core 7.60 all custom class names are being ignored and a default name is substituted.
Views preview mode however, the custom classes are rendered correctly.
ie
views preview
<div class="project views-fieldset" data-module="views_fieldsets">
renders on page
<div class="views-fieldsets-div">

Steps I've taken so far

Basic drupal 7.60 install with only Views fieldsets and required modules. Works fine.

My current production site in 7.59 backup and restore works as expected.

With 7.60 with previous database restore, removes custom class names after cache clear.