adaldesign on "meta_query 'compare' =>'IN' not working as described in Codex"

I'm at a loss, trying to use what seemed like a rather textbook meta_query parameter as one of the arguments in a get_users() request.

I formatted the code (displayed below) using the instructions provided on the get_users Codex page, as well as the WP_Meta_Query Codex page referenced.
There is also a post on StackOverflow that suggests this same syntax should work just fine.

However it's not working for me. I am able to get results using the 'LIKE' compare key but that one can't process an array of values like 'IN' is supposed to. I could try to hack my way around this with an ugly foreach loop but I'd really rather keep it kosher.

Here's my code:

$user_query_args = array(
	'role'         => '',
	'meta_query' => array(
		array(
			'key' => 'guest_type',
			'value' => $gest_type // this works fine
		),
		array(
			'key' => 'wp_'. $blog_ID .'_registration_status',
			'value' => $registration_status // this works fine
		),
		array(
			'key' => 'mandatory_tracks',
			'value' => 'newphy',
			'compare' => 'IN'  //*** but this don't work ***
		)

	),
	'orderby'      => 'login',
	'fields'       => 'ID'
);

$reported_users_IDs = get_users( $user_query_args );

And just so you know I'm not crazy, this row exists in the user_meta table of the database:

82 3 mandatory_tracks a:2:{i:0;s:6:"newphy";i:1;s:9:"workforce";}

Finally, you can see my work in progress with some var_dumps here.

Any help is greatly appreciated!