I've had magnetic cores before
It's possible the brown pair is the wrong spec core but everything just fits in the outer sheath so it's unlikely. My money is on it not actually being ethernet cable.
I can't even blame it on being a language problem because the electrician was shocked to see it too and the electrical wholesaler assured him the cable would be good.
If I can find out what it actually is and discover that it isn't ethernet cable, I can just ask the electrician to replace it.