But if it is faulty, it could be possible to change the configuration by turning the switch around and swapping conductors over to the opposite gang.
This could confirm that.
As it is, one of the yellows is the switched live.
As you say that the light does not go out when this switch is operated, that means the yellow is always live (except when the other switch is 'off') therefore this switch is not working as it should.
Edit - assuming the other switch is correctly wired and the lights worked properly before.
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