I an quite sure, that there's an interference between your template and JEM. Because JEM is fully compatble with the official joomla templates, you should consult the template makers of your template. You paid enough for it, so you should ask for help from them and not from the makers of our free extension. But when you got help from them, you are invited, to share the interestin results here. Thank you.
PHP 7.2.9 on different sites
JEM 2.3.0dev 2.0M and specials with Joomla! 3.9.3 stable on livesites
ja_simply, ja_purity_iii, multilingual DE EN IT FR