I had a very similar issue on one site that I manage. I was able to take some time recently to dig in and debug a bit after keeping the version below v3.3.6 for a couple of months.
It ended up being calls to apply_filter('the_content', $content);
in 3 Advanced Custom Fields block templates. I haven’t gone any further, and I determined that the the_content
filter wasn’t needed in those areas for my purposes.
It seems that a filter from NextGen was added around this time. Hopefully, this could help narrow down the issue for you guys.