The doc says

To get the contents of a collection, simple send a GET request to the releases resource
GET /ws/2/collection/f4784850-3844-11e0-9e42-0800200c9a66/releases

but for me it doesnt show any releases

<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#">
<collection id="7ba9bdcf-6476-4a34-99bb-9b927670ed38">
<name>My Collection</name>
<editor>ijabz</editor>
</collection>
</metadata>

even though there are releases as proofed by collection call

http://test.musicbrainz.org/ws/2/collection

<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#">
<collection-list>
<collection id="7ba9bdcf-6476-4a34-99bb-9b927670ed38">
<name>My Collection</name>
<editor>ijabz</editor>
<release-list count="1173" />
</collection>
</collection-list>
</metadata>