Something is wrong in your conversion, you are storing the entire uer object. What you want to store is just the information that you want to search by, for example the ID if that’s what you want to do the query on, or the persons name, or whatever. Then you can do
'meta_query = array( array( 'key' => 'related_trustee_wp', 'value' => array(1, 7, 2), 'compare' => 'IN' ) )
The problem with the repeater is not the meta_value, it is the meta_key. Because they are not all the same then you must to a “LIKE” query on the meta_key column. You can find information on doing this here https://www.advancedcustomfields.com/resources/query-posts-custom-fields/. This is what I refuse to do.
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