The first thing to do, before we try anything else, is to see if your hosting (WP Engine) is the issue.
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)