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Not sure if it’s related to my setup or the plugin or a WordPress update with changes to WP_Query but the s argument passed empty to WP_Query is causing the issue.

I created a function to hook to your WP_Query filter and that solves it:

function post_grid_filter_query_args($default_query = null) {
	
	unset($default_query['s']);
	
	return $default_query;
}
	
add_filter('post_grid_filter_query_args', 'post_grid_filter_query_args', 10, 1);

Glad to see you solved it.

Can you please check which plugin or theme causing the issue so we can update plugin to fix the issue.

Regards