I removed any reference to pre_get_posts the best I could. I’ve made my theme able to drop the multilingual plugin I have activated for testing and I believe I found the issue.
ACF 5.2.X is not compatible with PolyLang Version 1.7.2.
Check out polylang/frontend/frontend-auto-translate.php; PLL_Frontend_Auto_Translate->pre_get_posts($query);
https://wordpress.org/plugins/polylang/
