This is a misleading bug for users on OPAC, as they can think that a book is not currently available when it has still available items. If we cannot solve this bug, it would be better not to give this feature.
Confirmed in master. A title with multiple items, one of which is checked out, is hidden from search results when the 'Limit to available' filter is applied.
It looks like this was fixed 2019 with bug 11677. *** This bug has been marked as a duplicate of bug 11677 ***