ERA British Standard latch – these are often installed incorrectly by builders. Scarily often. If they left the cylinder screws out and only used the escutcheon screws to fasten it in place, the cylinder can sometimes rotate with any key. That’s more likely than the two locks being keyed the same.
Edit: I’ve just seen the filler! £10 and a bag of smarties says that the builder couldn’t get it lined up properly so chucked out or loosened some screws
Easy test to do to find out, but hard to explain. Give me 10 mins and I’ll make a video.