Paying in Cash

I seem to remember Starling saying that their new Teal cards would work at the post office but their old Purple (and much nicer ones) would not. Something do with chip (again IIRC)

Edit:
Also mentioned here Starling Discussion & Feedback - #1382 by chrisgreen and here Introducing: Cash deposits at the Post Office - Starling Bank

No idea what the technical reason is though