Data points -

I was able to do 2 mixed cabin redemptions in December 2021 by calling in.

1) 1st booking - LH First combined with LH business
2) 2nd booking - TK business combined with TK economy

It took me only 2 tries and the agent on the 2nd attempt knew exactly what to do.

I called SIN based contact center, which I advise despite its horrendous wait times currently.