How to query posts by meta keys AND under specific category?

Are you using a checkbox or similar for the ‘acf_platform’ field using ACF for more than one selection?

Advanced Custom Fields stores these as a serialised array, so a meta_query with ‘compare’ => ‘=’ may not work.

The ACF documentation suggests using ‘compare’ => ‘LIKE’

$args = array(
   'meta_query' => array(
         array(
            'key' => 'field_name', // name of custom field
            'value' => '"red"', // matches exaclty "red", not just red. This prevents a match for "acquired"
            'compare' => 'LIKE'
        )
   )
);

http://www.advancedcustomfields.com/resources/checkbox/

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EDIT: Actually, I realised that if this was the case your query should be returning no results. You mentioned it returned both posts, so this may not be the case.