This is not really a bug per se, but a backward incompatible change that affects current Picard installations and possible other software.

On the current live server, if you query the webservice for a release with the inc=collections parameter and you don't have this release in any of your collections, the <collection-list> node will be omitted:

http://musicbrainz.org/ws/2/release/cd36288f-8a82-42c2-91d1-1344c69168c3?inc=collections

If you do the same on the current beta, an empty "<collection-list count="0"/>" will be part of the result:

http://beta.musicbrainz.org/ws/2/release/cd36288f-8a82-42c2-91d1-1344c69168c3?inc=collections

I would advise in fixing this, even though this is arguably a bug in Picard (I will make sure it gets fixed there, too). But if this code goes live it will affect a large amount of users, and rolling out a new Picard release is, despite us releasing Picard not very often, a slow process nevertheless.