Before we dig any further in to any issues raised, the first thing to do, before we try anything else, is to see if your hosting (WP Engine) is the issue (I do not think that it is, but best to make sure).
There is a setting to override a ‘feature’ of WP Engine hosting, that truncates long SQL queries (which ours are). Edit the site wp-config.php file and add this at the top, after line 1 (on a new line 2):
define('WPE_GOVERNOR', false);
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(also see here for another related support note about this – https://support.searchandfilter.com/forums/topic/killed-query-errors-in-log/#post-64826)