I'm sorry to say there is a bug in the ISBN, ISSN, and LCCN SRU queries. They do not put results in a FRBRgrouping. The results of any ISBN, ISSN or LCCN query will get you all bibliographic records when it should give you the default result of FRBRgrouping=on which tends to be a single record, even with an SRU query. When the system is working correctly, there are times when there may be more than one edition using the same ISBN, but it isn't that common.
All other SRU queries does work correctly.
We will look into what could have caused this and get it resolved as quickly as possible.
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