I studied at Cambridge, and I have _never_ heard "Master's in X" used to describe the MA (Cantab.). It's simply wrong - the MA is not a subject specific qualifications - and can be trivially checked by employers - neither degree certificate (the BA or MA) states subject[1], and the degree transcript will indict you only took undergraduate level courses. As a result, the phasing is typically "I have an undergraduate degree in X"/"I did my undergrad. at Cambridge".
If people are using it to be misleading, that isn't really Cambridge's problem -- anywhere it matters, people can ask for proof (the transcript), and there is a fair chance the person is being misleading in other ways as well.
[1] Example BA certificate: https://i.stack.imgur.com/VTWUb.png All undergraduate degrees are the Bachelor of Arts, regardless of subject (except Law, which is the LL.B.).
If people are using it to be misleading, that isn't really Cambridge's problem -- anywhere it matters, people can ask for proof (the transcript), and there is a fair chance the person is being misleading in other ways as well.
[1] Example BA certificate: https://i.stack.imgur.com/VTWUb.png All undergraduate degrees are the Bachelor of Arts, regardless of subject (except Law, which is the LL.B.).