Not a calibre bug after all.
Equality searches on identifiers have a special syntax:
- identifiers:=isbn:123 will find books with a type equal to isbn having a value containing 123.
- identifiers:=isbn:=123456789 will find books with a type equal to isbn having a value equal to 123456789.
http://manual.calibre-ebook.com/gui....arch-interface
I'll release a corrected version of FFDL PI shortly.