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WordPress security guidelines require sanitizing all HTML output so there’s a function in framework/init.php wp_kses_wf() around line 130 which handles that.
Try adding another tag block for iframe with all the attributes you need like src. Something like:$allowed_tags['iframe'] = array(
'style' => true,
'class' => true,
'id' => true,
'data-*' => true,
'src' => true,
);