Were you logged in as admin while reporting?

Contributor should not be the issue.

When you clicked the submit button on the reporting form, what message did it show? Are you sure the posts were generated by the user in the list.

Can you please login as one of the white listed user and post an activity and then try to report it(by submitting the form) from another account.

Regards
Brajesh

Hi Tosin,
I am sorry for the inconvenience.

There was a small bug in the code for stopping the profile of a user from being related(it should return false not true).
I am still not sure why your content is not being white listed.

Here is the updated code(only 2nd part has 1 line changed)

// BuddyPress Moderation Tools: White list specific user's content from moderation.
add_filter( 'bpmts_is_user_whitelisted', function ( $is, $user_id ) {
	$white_listed_users = array( 284, 37845, 4578, 2357 );
	if ( in_array( $user_id, $white_listed_users, true ) ) {
		$is = true;
	}

	return $is;
}, 10, 2 );

// BuddyPress Moderation Tools: Do not allow white listed user's profile to be reported.
add_filter( 'bpmts_is_item_reportable', function ( $is, $item_type, $item_id ) {
	if ( 'user' !== $item_type ) {
		return $is;
	}

	$white_listed_users = array( 284, 37845, 4578, 2357 );
	if ( in_array( $item_id, $white_listed_users, true ) ) {
		$is = false;
	}

	return $is;
}, 10, 3 );

Please try reporting a user now.

Regards
Brajesh

Is there any chance you are using any filter/custom code related to moderation tools other than the above?

Hi Tosin,
Thank you for the details.

I am sorry but it seem strange. I did not test the code(though I am pretty sure it works). Please allow me to check it today and get back to you by early tomorrow.

Regards
Brajesh