Unfortunately same problem with web service.
Click the JSON link, press F5 several times, you will see a different subset of 25 items among 156 total, not always the same (first item will alternate between Things Have Changed and Melancholy Mood, same for the others).
You cannot paginate this result set to get all items without duplicate and without missing any.
So this bug (MBS-12154) is also happening with release groups:
Thanks @aerozol for reminding me this topic.
Web service query search results should be ordered by entity row ID to ensure a working pagination with offset and limit, as documented.