FCL.620 IR -Skills Test[...]
(c) Applicants who have completed a skill test for a multi-engine IR in a single-pilot multi-engine aeroplane for which a class rating is required shall also be issued with a single-engine IR for the single-engine aeroplane class or type ratings that they hold.
The rating, SEP(land), wasn't held at the moment of the skills test, so the CAA shouldn't have issued it as valid for SE. No doubt it didn't fit their checklists
As you technically never held the IR-SP-SE, it should probably be training at an ATO the an IR-SP-SE skills test. However because the CAA put it on your licence you could probably get away with cross crediting, fly 3 approaches in a SEP with an IRI, show it to an IRE and they can sign the IR-SP-SE off with the same validity date as your IR-SE-ME. Next year you can then sign them both off with the same profcheck.