The searchable field "orderingattribute" of series is set to "number", that is the name of the relationship attribute type https://musicbrainz.org/relationship-attributes#number). It is filled with an unique value, so it is useless as-is. It isn’t being used either. And it was even unintended according to bitmap (see chat logs):
orderingattribute ultimately wasn't intended to be searchable, but I think was left in on accident or in case it was used in the future
originally series were designed to support different ordering attributes but we decided to just have one after the schema change (or maybe before but it was too late to change the sql)
see https://chatlogs.metabrainz.org/brainzbot/musicbrainz-devel/2014-05-13/?msg=45564&=&page=1 too
There is no more plan to add other values than "number"" for ordering value, so there is no point to keep it as search field.