Filter hooks not working

[iaincoughtrie]

Attempting to update the properties sent to Segment via any of the documented filters does not appear to be working.

Basic example:

add_filter( 'segment_for_wp_change_event_properties', 'modify_event_properties', 10, 3 );

function modify_event_properties( $properties, $action, $data ) {
	$properties['custom_property'] = 'Custom Value';

	return $properties;
}

This does not result in the custom property being logged with the events in Segment.