I had a bug with the ez_render_field in my twig file described here : http://share.ez.no/forums/ez-publish-5-platform/display-xml-block
After some digging in the vendors, I found the source of its misbehavior.
The thing is happening in vendor\ezsystems\ezpublish-kernel\eZ\Publish\Core\FieldType\XmlText\Converter\Html5.php in the getXSLTProcessor function.
In fact, if the foreach try to load a nonexistent customTag.xsl listed in the ezpublish.yml, all the ez_render_field will fail all the render.
So, I think it could be a good point to put a condition to check if the stylesheet exists or not just before making your magic.