Just for record for the future users.
I tried everything you said before, but I’m allowing multiple values at the relational user field. The only way it works was as I found on this stack: https://wordpress.stackexchange.com/questions/55354/how-can-i-create-a-meta-query-with-an-array-as-meta-field.
Since the meta_value is a serialized array (on my case), I had to handle it a little differently:
'meta_query' => array(
array(
'key' => 'my_meta_key',
'value' => serialize( strval( get_current_user_id() ) ),
'compare' => 'LIKE'
)
)
